Add minimal CONTRIBUTING.md.

CONTRIBUTING.md

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+Whenever you create a Pull Request (PR), a maintainer will self-assign
+themselves as the *upstream*. The upstream decides if your PR is accepted and
+might require you to amend additional changes before merging. You can expect
+the upstream to communicate clearly if there are any issues preventing your PR
+from being merged, and how they can be fixed. Once merged, the upstream will
+add the `merged` label to your PR.
+
+You can also submit PRs that are not ready to be merged: include the labels
+`[wip]` in the title of PRs that require further work, and `[sketch]` for PRs
+that are not meant to be merged at all.
+
+Please cross reference related issues and PRs extensively by mentioning them in
+descriptions or comments (#XXX), as it helps maintainers to read up on related
+information.
+
+Our [Documentation Guide](https://github.com/SnabbCo/snabbswitch/blob/master/src/doc/documentation-guide.md)
+gives pointers on how to contribute to the project's documentation.
+
+Copyright notices belong into `src/COPYRIGHT`; copyright notices in other files
+will be rejected.