docs: add project documentation

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in improving htop-web.
+
+## Before making changes
+
+1. Review the README and relevant documentation.
+2. Check the existing implementation before introducing new dependencies or changing behavior.
+3. Keep changes focused on a clear issue or improvement.
+
+## Development workflow
+
+1. Create a branch for your change.
+2. Implement and test the change.
+3. Run the backend Gradle build from `backend/`:
+
+   ```bash
+   ./gradlew clean build
+   ```
+
+4. Review the Git diff and ensure generated files, credentials, and local configuration are not included.
+5. Update documentation when necessary.
+6. Submit your changes with a clear description of the problem and solution.
+
+## Code quality
+
+- Follow the conventions already used in the relevant module.
+- Avoid unrelated refactoring.
+- Do not claim support for monitoring capabilities that have not been tested in the target environment.
+- Never commit passwords, tokens, private keys, or production secrets.
+
+## Reporting issues
+
+Include the steps to reproduce the issue, expected behavior, actual behavior, relevant logs, and environment details. Remove sensitive information from logs before sharing them.