docs: add contributing guide

docs/CONTRIBUTE.md

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+# Contributing
+
+Thanks for your interest in MimiClaw. This project welcomes focused, high-quality contributions that improve reliability, performance, or documentation.
+
+## Scope
+
+We accept contributions in these areas:
+
+- Core firmware features and bug fixes
+- Documentation, examples, and diagrams
+- Build and tooling improvements
+- Tests and CI enhancements
+
+> **外设**
+> We are not accepting PRs that add or change external peripherals right now.
+
+## Before You Start
+
+- Search existing issues and discussions to avoid duplication.
+- Open a short proposal for large or risky changes.
+- Keep changes small and reviewable when possible.
+
+## Development Setup
+
+- Install ESP-IDF v5.5+.
+- Build targets are in `idf.py`.
+- Default config lives in `main/mimi_secrets.h.example`.
+
+## Branching and Commits
+
+- Use a short, descriptive branch name.
+- Keep commit history clean and focused.
+- Suggested commit style: `docs: ...`, `fix: ...`, `feat: ...`.
+
+## Pull Requests
+
+- Describe the problem and the solution clearly.
+- Include testing steps and results.
+- Update documentation when behavior changes.
+
+## Code Style
+
+- Match existing naming and formatting.
+- Prefer clarity over cleverness.
+- Avoid large refactors mixed with functional changes.
+
+## Tests
+
+- Add or update tests when behavior changes.
+- If tests are not available, explain why and how you validated the change.
+
+## Documentation
+
+- Keep README and docs in sync with behavior changes.
+- Add concise examples for new features.
+
+## License
+
+By contributing, you agree that your contributions will be licensed under the MIT License.