Add README.md with rpm-build instructions rockstor#2

Initial draft of developer focused About/Howto README.md.
Intended to enable contributors to do DIY RPM builds
for end-to-end testing of proposed changes in related
repositories. Based on using the official Rockstor
installer to promote self-hosting capabilities.

README.md

@@ -0,0 +1,133 @@
+# Howto rpmbuild the 'rockstor' package
+
+## Repository contents
+
+This repository [rockstor-rpmbuild](https://github.com/rockstor/rockstor-rpmbuild) contains the rpmbuild spec file,
+the repositories license file: `SPDX-License-Identifier: GPL-3.0-or-later`
+FSF Free/Libre, & OSI approved according to: [SPDX License List](https://spdx.org/licenses/).
+And this README.md: to be formatted according to 
+[Semantic Linefeeds](https://rhodesmill.org/brandon/2012/one-sentence-per-line/)
+which sites Brian W. Kernighan (1974).
+I.e.: For content additions/changes please stick to **one sentence per line**
+as this helps with translations and reviews of changes.
+Really long sentences may be broken at punctuation points.
+
+## Contributions
+
+As with all 'The Rockstor Projects' repositories, contributions are always welcome.
+We depend upon domain experts to assist with our community endeavour.
+For contribution guidelines, see:
+[Contributing to Rockstor - Developers](https://rockstor.com/docs/contribute/contribute.html#developers).
+
+## Getting started
+
+This repository is aimed at developers/enthusiasts only.
+See our [Regular documentation](https://rockstor.com/docs/) for general infomation about Rocksor,
+and specifically the [Installation]() subsection for how to install a Rockstor instance.
+The rest of this guide assumes such an instance is in use, and has been fully updated.
+
+Using an existing Rockstor install ensures that at least our prior dependencies are in-place.
+This helps with establishing if any new dependencies are required when making spec file changes.
+The rpmbuild process, within the %check scriptlet, also runs all our existing unit tests.
+As such a properly configured and running Postgres server is required.
+The prior installs rockstor-pre.service (initrock) would have asserted these conditions.
+Other-wise the rpmbuild command will fail on the %check scriptlet stage. 
+
+### Remove the exising package
+
+See: [Remove the Existing Rockstor RPM install](https://rockstor.com/docs/contribute/contribute.html#remove-the-existing-rockstor-rpm-install)
+
+### Establish rpm spec file
+
+The RPM spec file named **rockstor.spec** is required on the prior installer instance.
+
+#### Via your rockstor-rpmbuild GitHub fork
+
+It is assumed in this section that you have forked the rockstor-rpmbuild GitHub repository,
+as per the Developer docs advise for the rockstor-core repository.
+
+```shell
+zypper --non-interactive install --no-recommends git
+cd /opt
+git clone https://github.com/your_github_username/rockstor-rpmbuild.git
+cd rockstor-rpmbuild/
+git checkout testing  # or stable, or your issue branch
+```
+For specific git branches re making/submitting back changes see the
+[Making changes](https://rockstor.com/docs/contribute/contribute.html#making-changes)
+doc section and adapt for this rockstor-rpmbuild repository.
+
+If you are developing a contribution, be sure to include all upstream changes:
+in the case of developing against the **testing channel** (normal dev target) use:
+```shell
+git remote add upstream https://github.com/rockstor/rockstor-rpmbuild.git
+# git remote show  # should then indicate all remotes.
+git pull --rebase upstream testing
+```
+
+#### Via rsync from your development system
+
+Or you could rsync your local copy of the rockstor-rpmbuild repository, over from your development system.
+
+## Install rpmbuild
+```shell
+zypper install --no-recommends rpm-build
+```
+**NOTE: Solution 1: deinstallation of busybox-gzip ...**
+May be required (replaced by the full gzip).
+See: [pre-install gzip to avoid busybox-gzip via dependency](https://github.com/rockstor/rockstor-installer/issues/135)
+
+
+## Build the RPM/SRPM
+
+Both the regular RPM, and it's source variant (SRPM) can be build with a single command.
+
+```shell
+# Current dir must be that of the spec file:
+rpmbuild -ba -v rockstor.spec
+```
+
+The tail end of the resulting output indicates the built RPM and SRPM files, e.g.:
+```shell
+...
+Wrote: /usr/src/packages/SRPMS/rockstor-4.5.7-0.src.rpm
+Wrote: /usr/src/packages/RPMS/x86_64/rockstor-4.5.7-0.x86_64.rpm
+...
+```
+
+Note that currently our arch specific RPMs are built to prove the integrated (%check) tests,
+on the intended/target architecture.
+Their contents, for now at least, are not actually architecture specific.
+But when installed, the resulting RPMs' Poetry build orchestrator,
+links/downloads and/or builds what is found to be required,
+and places it in the .venv subdirectory of /opt/%{name}
+
+## Caveats
+
+### RPM will not be signed.
+
+All official Rockstor packages, and their host repositories, are singed by the project.
+As such a hand/custom-built package will have no signature.
+Options:
+1) The fix is to sign you rpm.
+2) The workaround (reasonable for development testing) is to ignoe gpg checks via e.g.:
+    ```shell
+    zypper --no-gpg-checks /usr/src/packages/RPMS/x86_64/rockstor-4.5.7-0.x86_64.rpm
+    ```
+
+### RPM relocation
+
+RPM Relocation; see: "Prefix: /opt" in the spec file, is known to be incompletely implemented:
+both here in [rockstor-rpmbuild](https://github.com/rockstor/rockstor-rpmbuild)
+and in the [rockstor-core](https://github.com/rockstor/rockstor-core).
+Contributions to complete this capability are of course welcome.
+But this is not a priority for the project.
+
+### Package renaming
+
+There is facility within the rpmbuild spec file to rename the resulting package.
+This, as per RPM relocation above, is also only partially implemented across our repositories.
+And would entail far more work to implement than the relocation capability.
+Again this is not a priority for the project.
+
+Both RPM relocation and renaming would benefit from package-name/install-location aware database instances.