Drops a cheat sheet (CONTRIBUTING.md) in repo root so anyone — you,
future maintainers — has the full commit-prefix table one glance
away. Covers which prefixes trigger a release and which don't.
Also adds .gitmessage that you can optionally wire into git as the
default commit template:
git config --local commit.template .gitmessage
Opens the cheat sheet in your editor every time you `git commit`
without -m. Remove it with `git config --local --unset commit.template`.
This commit uses `docs:` prefix so it does NOT trigger a release —
proving the auto-version workflow's filter works.
