add contributing.md file

CONTRIBUTING.md

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+# CONTRIBUTING.md
+
+Thank you for contributing to Supabase Agent Skills! Here's how to get started:
+
+[1. Getting Started](#getting-started) [2. Issues](#issues)
+[3. Pull Requests](#pull-requests)
+[4. Contributing New Rules](#contributing-new-rules)
+[5. Extending Existing Skills](#extending-existing-skills)
+
+## Getting Started
+
+To ensure a positive and inclusive environment, please read our
+[code of conduct](https://github.com/supabase/.github/blob/main/CODE_OF_CONDUCT.md)
+before contributing.
+
+## Issues
+
+If you find a typo, have a suggestion for a new skill/rule, or want to improve
+existing skills/rules, please create an Issue.
+
+- Please search
+  [existing Issues](https://github.com/supabase/agent-skills/issues) before
+  creating a new one.
+- Please include a clear description of the problem or suggestion.
+- Tag your issue appropriately (e.g., `bug`, `question`, `enhancement`,
+  `new-rule`, `new-skill`, `documentation`).
+
+## Pull Requests
+
+We actively welcome your Pull Requests! Here's what to keep in mind:
+
+- If you're fixing an Issue, make sure someone else hasn't already created a PR
+  for it. Link your PR to the related Issue(s).
+- We will always try to accept the first viable PR that resolves the Issue.
+- If you're new, we encourage you to take a look at issues tagged with
+  [good first issue](https://github.com/supabase/agent-skills/labels/good%20first%20issue).
+- If you're proposing a significant new skill or major changes, please open a
+  [Discussion](https://github.com/orgs/supabase/discussions/new/choose) first to
+  gather feedback before investing time in implementation.
+
+### Pre-Flight Checks
+
+Before submitting your PR, please run these checks:
+
+```bash
+npm run validate  # Check rule format and structure
+npm run build     # Generate AGENTS.md from rules
+```
+
+Both commands must complete successfully.
+
+## Questions or Feedback?
+
+- Open an Issue for bugs or suggestions
+- Start a Discussion for broader topics or proposals
+- Check existing Issues and Discussions before creating new ones
+
+## License
+
+By contributing to this repository, you agree that your contributions will be
+licensed under the MIT License.