docs: add CONTRIBUTING.md guide

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in contributing to HospitalSanitizationTracker!
+
+## How to Contribute
+
+1. Fork the repository
+2. Create a feature branch: `git checkout -b feat/your-feature`
+3. Commit your changes: `git commit -m 'feat: add some feature'`
+4. Push to the branch: `git push origin feat/your-feature`
+5. Open a Pull Request
+
+## Code Style
+
+- Follow [Solidity Style Guide](https://docs.soliditylang.org/en/latest/style-guide.html)
+- Write tests for new contract functions
+- Keep frontend JS clean and commented
+
+## Running Tests
+
+```bash
+npx hardhat test
+```
+
+## Reporting Issues
+
+Open a GitHub Issue with a clear description and steps to reproduce.