docs: add limits and contributing guidance

[sdhilip200]

Adds a short Limits section to make clear that detect mode flags writing patterns, not authorship.

Also adds CONTRIBUTING.md with a small review checklist for future rule and docs changes.

Changes:

• clarify that the skill is not an AI detector
• warn against using findings to accuse someone of using AI
• add contribution guidance for new patterns, examples, and eval updates

[petergyang]

Thanks. The CONTRIBUTING.md checklist is useful, while the README limits are now covered by the current detect-mode language and the README has since been rewritten. Could you rebase on current main and narrow this PR to an updated CONTRIBUTING.md with the current skills/no-ai-slop/ paths?

[sdhilip200]

Rebased onto current main and narrowed the PR to CONTRIBUTING.md only. I dropped the README change and updated the guide to point at the current repo paths under skills/no-ai-slop/. The branch now differs from main only in CONTRIBUTING.md.