Merge pull request #1343 from l0b0/docs/pr-template-checklist

docs: Contribution checklist as part of PR template

.github/PULL_REQUEST_TEMPLATE.md

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+[Contribution](README.md#contributing) checklist (recommended but not always applicable/required):
+
+- [ ] There's an _[automated test](tests/default.nix)_ for this change
+- [ ] Commit messages or code include _references to related issues or PRs_ (including third parties)
+- [ ] Commit messages are _[conventional](https://www.conventionalcommits.org/)_ - examples from [the log](https://github.com/nix-community/poetry2nix/commits/master) include "feat: add changelog files to fixup hook", "fix(contourpy): allow wheel usage", and "test: add sqlalchemy2 test"