Create CONTRIBUTING.md

added initial advice for code contributions

CONTRIBUTING.md

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+# Requesting new features or reporting bugs:
+
+To request a new feature, or report a bug, please create a new issue [https://github.com/brandmaier/onyx/issues]. Please search through existing issues first in order to avoid duplicated issues.
+
+# Contributing to Onyx
+
+Anyone is welcome to contribute to Onyx. If you have fixed a bug or implemented a feature on your own branch, please submit a pull request. Pull request must be reviewed by either Timo or Andreas before being merged into the main branch. 
+Code submissions must be provided under the same license as the Onyx source code, which is the Apache 2.0 license.
+