First take on a contributing guide for `ty`. Lots of it is copied from
the existing Ruff contribution guide.
I've put this in Ruff repo, since I think a contributing guide belongs
where the code is. I also updated the Ruff contributing guide to link to
the `ty` one.
Once this is merged, we can also add a link from the `CONTRIBUTING.md`
in ty repo (which focuses on making contributions to things that are
actually in the ty repo), to this guide.
I also updated the pull request template to mention that it might be a
ty PR, and mention the `[ty]` PR title prefix.
Feel free to update/modify/merge this PR before I'm awake tomorrow.
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Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>
Co-authored-by: David Peter <mail@david-peter.de>
