Add some contributor/developer guidelines (#5399)

* Add some contributor/developer guidelines

[skip ci]

* Apply suggestions from code review

Co-authored-by: James Mitchell <james-d-mitchell@users.noreply.github.com>

---------

Co-authored-by: James Mitchell <james-d-mitchell@users.noreply.github.com>

CONTRIBUTING.md

@@ -182,7 +182,7 @@ This section is based on <https://lornajane.net/posts/2015/code-reviews-before-y
   * If you have cloned the PR use `git log --stat`.
 * Eyeball the diff for
   * Large commented / unused sections of code
-  * Strange variable or function names 
+  * Strange variable or function names
   * Duplicate code
 
 ### Dive into the code
@@ -197,6 +197,16 @@ This section is based on <https://lornajane.net/posts/2015/code-reviews-before-y
   If not, label the issue / PR accordingly.
 
 
+## Contact
+
+GAP uses Slack, a multi user chat system, for discussions. Everyone can
+join via a web browser or a dedicated client (e.g for Android or iOS).
+Just follow the link <https://gap-system.org/slack>.
+
+There is also an open developer list with currently very low volume.
+You can subscribe to it at <https://lists.uni-kl.de/gap/info/gap>.
+
+
 ## Additional Resources
 
 * [GAP Tutorial](https://www.gap-system.org/Manuals/doc/tut/chap0_mj.html)

DEVELOPING.md

@@ -0,0 +1,198 @@
+# Developer guidelines
+
+This document contains information potentially relevant for GAP developers.
+Please also check out the file `CONTRIBUTING.md` for getting started working
+on GAP. In contrast, this file here contains more specialized information that
+you can use like a reference manual.
+
+
+## Branches
+
+At any time, there are two primary branches in the GAP git repository:
+
+- `master`: all new changes go into this; pull requests usually target it
+- `stable-4.X` (where `X` is some number): stable releases of the
+  `4.X.*` series are made from this branch
+
+We are using GitHub's "branch protection" feature to restrict direct pushes
+to these branches. All changes should go through pull requests, to ensure
+that tests are run automatically and are passing (hard experience taught us
+that even "trivial" and "obviously correct" changes can break things).
+
+Note that we only make releases from the most recent release branch; older
+release branches are kept around but we do not expect to e.g. make a new
+GAP 4.12.x release after 4.13.0 has been released, and so on.
+
+
+### Making changes to the release branch via backports
+
+By default all changes go into the `master` branch. Once a release 4.X.Y
+is out, to get changes into the 4.X.(Y+1) release, the usual process is
+to first land them into the `master` branch, and have them "backported"
+later on the `stable-4.X` branch.
+
+The process for backporting is as follows:
+1. someone adds the `backport-to-4.X` label to the pull request (if you
+   don't have permission, just ask for someone, e.g. @fingolfin, to add
+   the label)
+2. some time after the PR has been merged into `master`, and before the
+   next stable release, someone will go through the list of merged PRs
+   with this label, and backport them (usually by opening a dedicated
+   backporting PR, which may contain the content of multiple PRs being
+   backported).
+3. After a PR has been backported this way, the `backport-to-4.X` label
+   is replaced by a `backport-to-4.X-DONE` label, to avoid
+   processing it again (this is also important for our scripts which generate
+   the changelog updates for releases).
+
+
+### Making changes to the release branch directly
+
+Sometimes changes really are only for `stable-4.X` and not for `master`,
+usually because the two branches diverged sufficiently (e.g. code may have
+been rewritten in a major way on `master` which does not lend itself to
+backporting, but we still want to fix a bug in the old implementation for
+the release branch). In that case, you can create a pull request which
+directly targets the `stable-4.X` branch (the target
+branch can be selected on GitHub during the creation of pull requests). When making such a pull request, it's
+a good idea to point it out (and the reason) in the PR description.
+
+
+## Code formatting
+
+Some simple rules for formatting code
+- no tabs
+- no trailing whitespace
+- all source files should end with a newline
+
+This is automatically done if your editor honours our `.editorconfig`
+file. See <https://editorconfig.org> for a list of editors supporting
+this and how to configure them.
+
+For historical reasons we have no particular rule for formatting code
+beyond the above. To avoid a chaotic mess, please adapt your
+contributions to the surrounding code. So if you insert something into a
+function that uses 2-space indentation, also use that.
+
+Please refrain from reformatting code as you edit it, as this makes it
+much harder to review your changes. If you really must reformat
+something, do so in a separate commit which only contains formatting
+changes.
+
+For completely new kernel code in C/C++, you can use `clang-format` to
+format it. If you edit existing code, you can try `git clang-format`
+(which reformats only code that is already modified). But here, too,
+please avoid large-scale reformatting, and keep reformatting in separate
+commits. The following command can sometimes help format just the code
+you modified anyway:
+
+    git clang-format $(git merge-base master)
+
+
+## Continuous integration (CI)
+
+For every commit to our primary branches, and also for all pull requests, we
+use [GitHub Actions][GHA] to run the GAP test suite, in order to detect
+regressions early.
+
+Which tests are run is controlled by the files `.github/workflows/CI.yml` and
+`.github/workflows/release.yml`. The syntax for these files and more is explained
+in the [GitHub Actions documentation][GHA].
+
+See also `dev/ci.sh` and other `dev/ci-*.sh` files for the scripts that are
+being run as part of these CI tests.
+
+
+## Discussions
+
+GAP uses Slack, a multi user chat system, for discussions. Everyone can
+join via a web browser or a dedicated client (e.g for Android or iOS).
+Just follow the link <https://gap-system.org/slack>.
+
+There is also an open developer list with currently very low volume.
+You can subscribe to it at <https://lists.uni-kl.de/gap/info/gap>.
+
+
+## Pull requests
+
+Contributions to GAP should be done in the form of GitHub pull requests.
+Then our test suite (see "Testing" below) is automatically run as part of
+the "Continuous integration (CI)" tests (again see elsewhere in this file
+for details).
+
+By default, your pull requests should target the `master` branch (see the
+section "Branches" for details).
+
+We use labels on branches to help us generated release notes for new GAP
+versions. If you are a new contributor, don't worry about that, we'll take
+care of it (in fact you won't even have the access rights to set them --
+again, don't worry about that).
+
+If you do have the permissions to set labels, here are some guidelines:
+1. Always add one of the "release notes" labels.
+2. If the change is meant to be backported to the stable branch,
+   add a "backports" label (see elsewhere in this document for details).
+3. Consider adding one of the "kind" label to indicate whether the change
+   is a bug fix, enhancement or whatever else.
+4. Other labels are mostly optional and can help categorize things for
+   the release notes. You are invited to set them but it's not a must.
+
+Some other notes for pull requests
+- please read the section on "Code formatting"
+- try to focus your pull request, i.e. ideally don't change multiple
+  unrelated things in one PR (this makes reviewing your changes easier).
+
+
+## Releases
+
+For details on how GAP releases are made, see `dev/releases/README.md`.
+
+
+## Testing
+
+GAP includes a test suite inside the `tst` directory, which is split into
+multiple parts.
+
+- `testinstall`: quick test suite that one can test frequently.
+   Run via  `./gap tst/testinstall.g`
+- `teststandard`: slower but more comprehensive test suite.
+   Run via  `./gap tst/testinstall.g`
+- `testextra`: very slow test suite that tests even more
+   Run via  `./gap tst/testextra.g`
+- `testbugfix`: additional small test files that are created to verify
+   specific bugfixes work as intended
+   Run via  `./gap tst/testbugfix.g`
+- `testinstall`: quick test suite that one can test frequently.
+   Run via  `./gap tst/testinstall.g`
+
+
+## Tracking down regressions
+
+The script `dev/bisect.sh` can be very helpful in tracking down which commit
+introduced a regression. Please read the extensive comments in the file to
+learn what it does and how to use it. 
+
+
+## Vendored dependencies
+
+GAP contains copies of some third party software, mostly to make it easier to
+build GAP "out of the box", with its minimal dependencies included.
+GAP maintainers may wish to update these dependencies from time to time.
+
+- `extern/gmp`: copy of [GMP](https://gmplib.org), the GNU Multiple Precision
+  Arithmetic Library
+- `extern/zlib`:  copy of [zlib](http://zlib.net), a compression library used to
+  implement transparent reading and writing of .gz compressed files.
+- `hpcgap/extern/gc`: copy of the [Boehm garbage collector](https://www.hboehm.info/gc/)
+  used by HPC-GAP
+- `hpcgap/extern/libatomic_ops`: copy of `libatomic_ops`, a subproject of Boehm GC;
+  should thus be kept in sync with that.
+- `cnf/config.guess` and `cnf/config.sub`: used by our `configure` script.
+  Ideally `autoconf` or `autoreconf` should automatically install the latest
+  versions of these, but this doesn't happen (presumably due to a bug in `autoconf`).
+  They can be updated by running the `update-config.sh` script inside the `cnf`
+  directory.
+
+---
+
+[GHA]: https://docs.github.com/en/actions