Create CONTRIBUTING.md, lbr-stack/lbr_stack_doc#24

.github/CONTRIBUTING.md

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+The [LBR-Stack](https://github.com/lbr-stack/) thrives to build a friendly community for `KUKA` users to foster knowledge exchange! :)
+
+We **encourage** everyone to open issues for questions / problems / features. For additional information, we would like to provide some links below.
+
+### Knowledge Sources
+- [Open issues](https://github.com/lbr-stack/pyFRI/issues)
+- [Closed issues](https://github.com/lbr-stack/pyFRI/issues?q=is%3Aissue+is%3Aclosed)
+
+## Contributing
+### pyFRI
+Contributions are vital to this project. If you want to contribute a feature / fix proceed as follows
+
+1. Open an issue:
+  - Explain the issue and your solution
+  - Request a new branch: `dev-feature`, e.g. `dev-my-new-demo`
+3. Fork this repository
+4. Create a [pull request](https://github.com/lbr-stack/pyFRI/pulls) against `dev-<feature>`
+
+### New FRI version
+Refer [README](https://github.com/lbr-stack/fri#contributing) for instructions.