docs: add CONTRIBUTING.md (#119)

CONTRIBUTING.md

@@ -0,0 +1,35 @@
+# Contributing
+
+## Setup
+
+```bash
+git clone https://github.com/YOUR_USERNAME/next-ai-draw-io.git
+cd next-ai-draw-io
+npm install
+cp env.example .env.local
+npm run dev
+```
+
+## Code Style
+
+We use [Biome](https://biomejs.dev/) for linting and formatting:
+
+```bash
+npm run format   # Format code
+npm run lint     # Check lint errors
+npm run check    # Run all checks (CI)
+```
+
+Pre-commit hooks via Husky will run Biome automatically on staged files.
+
+For a better experience, install the [Biome VS Code extension](https://marketplace.visualstudio.com/items?itemName=biomejs.biome) for real-time linting and format-on-save.
+
+## Pull Requests
+
+1. Create a feature branch
+2. Make changes and ensure `npm run check` passes
+3. Submit PR against `main` with a clear description
+
+## Issues
+
+Include steps to reproduce, expected vs actual behavior, and AI provider used.