Add separate CONTRIBUTING.md

Author: dokempf

CONTRIBUTING.md

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+## Contributions
+
+This paper is a community effort and everybody who feels part of the German RSE community
+is invited to contribute.
+
+You can contribute in various ways:
+
+* Propose content/text via pull request
+* Improve existing content via pull request
+* Review or comment on pull requests of others
+
+Before contributing, please read the below rules
+
+### Process
+
+* Contributions should be submitted as **pull requests** to this repository on GitHub.
+* Pull requests that work on the content of the paper require at least **one review** to be merged.
+* Do not forget to add yourself to the list of authors
+
+### Style and formatting
+
+* We are following **British English** including e.g. "synergise" over "synergize".
+* Please add a **line break** after each sentence in the core.
+
+### Authorship
+
+We agreed to have two tiers of authorship:
+
+* Tier 1 authors contributed substantially to the paper through writing, review etc.
+* Tier 2 authors contributed at least once to the paper.
+
+All authors, regardless of the size of the contributions are mentioned in the author list.
+Tier 1 authors are mentioned before Tier 2 authors using the "equal contribution" marker.
+
+While contributors keep the copyright to their work, they agree to publish the text under **CC-BY 4.0**