📝 Add CONTRIBUTING.md with PR workflow documentation

- Comprehensive contributor guide (bilingual DE/EN)
- Explains automated PR build checks
- Local build & test instructions
- Code style guidelines
- PR checklist
- What contributions are welcome
- Linked in README.md and README.en.md

[skip ci]

README.en.md

@@ -65,6 +65,12 @@ cd android
 
 ---
 
+## 🤝 Contributing
+
+Contributions are welcome! See [CONTRIBUTING.md](CONTRIBUTING.md) for details.
+
+---
+
 ## 📄 License
 
 MIT License - see [LICENSE](LICENSE)