Rewrite the documentation part 1

CONTRIBUTING.md

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+# Contributing
+
+Thanks for contributing. This project is security-adjacent; changes should be deliberate and easy to review.
+
+## Basic workflow
+
+1. Fork the repository and create a feature branch:
+   ```bash
+   git checkout -b feature/<name>
+   ```
+
+2. Make changes with well described commits. (Run formatting and basic checks.)
+
+3. Open a pull request:
+
+   * Describe what changed and why
+   * Include screenshots for UI changes
+   * Include migration notes if you changed configuration/DB behavior
+
+## Coding and review expectations
+
+* Prefer readable code over "clever code".
+* Keep public-facing behavior documented (docs or inline help).
+* Do not introduce new external network calls by default; make them opt-in where possible.
+* For changes affecting auth or callback handling, include a short threat-model note in the PR description.
+
+## Reporting security issues
+
+If you believe you found a security issue, please do not open a public issue. Use a private disclosure channel appropriate for the project (for example, contact Swissmakers GmbH via website) and provide reproduction steps and impact assessment.