Release v5.0.1

docs: add CONTRIBUTING.md (#163)

CONTRIBUTING.md was written on the Booru branch (6d2b050) but pushed
after #161 was squash-merged, so it never landed on main. This brings
the commit across unchanged.

It points at the existing branch-and-PR workflow in the README and adds
rules for AI-assisted contributions: the contributor owns what they
submit, AI use is disclosed in the PR description, PRs implement agreed
changes rather than floating new ones, and one PR carries one concern.

AI use

Claude cherry-picked the original commit and wrote this description. The
file's contents are unchanged from 6d2b050 — no new prose in this PR.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Vac3uRQkYxtD2wE8QdTVSr

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Co-authored-by: Claude Opus 5 noreply@anthropic.com