Add a CONTRIBUTING file

CONTRIBUTING.rst

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+======================
+Contributing to gemato
+======================
+
+
+Policy
+======
+
+This project is developed under the umbrella of `Gentoo Linux`_.
+As such, it is bound by Gentoo policies, including but not limited to:
+
+- `Gentoo Social Contract`_
+- `Gentoo Code of Conduct`_
+- `Gentoo Copyright Policy`_ (GLEP 76)
+- `Gentoo AI Policy`_
+
+TL;DR:
+
+- be respectful
+- do not use AI tools
+- make sure you're legally allowed to submit anything you do under
+  the project's license (GPL-2+)
+- sign off your changes using your real name or an established online
+  identity
+
+
+The canonical repository
+========================
+
+The canonical `project repository`_ is hosted on git.gentoo.org,
+as ``proj/gemato.git``.  The project repositories on Codeberg and/or
+GitHub are read-only mirrors.
+
+
+Reporting bugs
+==============
+
+Bugs should be reported to `Gentoo Bugzilla`_.  You can use the handy
+URLs provided:
+
+- `report a bug / feature request`_
+- `report a security issue`_
+
+Please do not e-mail the maintainers or use Codeberg / GitHub comments
+to report issues with commits.
+
+
+Contributing
+============
+
+1. Do not use AI tools (why do I have to repeat that?!).  If you do,
+   your code will be tainted and we won't be able to accept it.  And no,
+   removing the label and pretending it's not slop won't help.
+
+2. Before submitting a major contribution or behavior change, please
+   report an issue and discuss your proposed changes first.  For minor
+   bug fixes, submitting the change immediately is fine.
+
+3. Please try to follow the existing coding style (preferably the one
+   from newer commits, sorry about the mess).  When in doubt, do what's
+   easier for you and we'll go from there.
+
+4. Test your changes using ``tox``.  You usually will also need to add
+   a test case, though we can help with that.
+
+5. Aim for clearly delineated commits with atomic changes and clear
+   commit messages.  Follow the existing style.  Ensure that all
+   significant information can be found in commits themselves, as pull
+   request description / cover letter will be discarded.  Become friends
+   with ``git rebase`` (see e.g. `git rebase in depth`_).
+
+6. It's fine (and even preferable for bigger changes) to send
+   work-in-progress changes for early feedback.  Do not hesitate to ask
+   for suggestions or help.
+
+7. If your changes aren't getting any attention, please ping us.  We may
+   have missed them, we may have gotten distracted, or we may simply not
+   have realized that they are ready.  Pinging on one of the `Gentoo IRC
+   channels`_ can be especially helpful.
+
+Changes can be submitted using:
+
+- pull requests to the `GitHub mirror`_ (preferable)
+- patches sent via e-mail to the maintainers
+- patches attached to bug reports
+
+
+.. _Gentoo Linux: https://www.gentoo.org/
+.. _Gentoo Social Contract: https://www.gentoo.org/get-started/philosophy/social-contract.html
+.. _Gentoo Code of Conduct: https://wiki.gentoo.org/wiki/Project:Council/Code_of_conduct
+.. _Gentoo Copyright Policy: https://www.gentoo.org/glep/glep-0076.html
+.. _Gentoo AI Policy: https://wiki.gentoo.org/wiki/Project:Council/AI_policy
+.. _project repository: https://gitweb.gentoo.org/proj/gemato.git/
+.. _Gentoo Bugzilla: https://bugs.gentoo.org/
+.. _report a bug / feature request: https://bugs.gentoo.org/enter_bug.cgi?product=Gentoo+Linux&component=Current+packages&short_desc=app-portage/gemato:+
+.. _report a security issue: https://bugs.gentoo.org/enter_bug.cgi?product=Gentoo+Security&component=Vulnerabilities&short_desc=app-portage/gemato:+
+.. _git rebase in depth: https://git-rebase.io/
+.. _Gentoo IRC channels: https://www.gentoo.org/get-involved/irc-channels/
+.. _GitHub mirror: https://github.com/gentoo/gemato/