Add CONTRIBUTING.md

Author: Rastislav Turanyi

CONTRIBUTING.md

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+# Contributing
+
+## Bugs and Features
+
+This project is generally feature-complete - it does one thing and hopefully does do well. However,
+if anything is not working as intended, please open an issue on our 
+[GitHub](https://github.com/pace-neutrons/inline_reference/issues) to discuss it.
+
+## Code contributions
+
+Code contributions to this project are welcome! Though, please open an issue on our 
+[GitHub](https://github.com/pace-neutrons/inline_reference/issues) first to discuss it, after which
+you can fork the repository and open a pull request.