Add CONTRIBUTING.md

Author: ufechner7

CONTRIBUTING.md

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+## How to contribute to this project
+
+### Casual contributors
+
+#### Create issues
+1. mention the version number of the package
+1. write at least one sentence describing the issue
+1. share the output of `versioninfo()`
+1. indicate if it is a bug report or feature request
+
+### Regular contributors
+1. become member of the Telegram channel
+1. ask for write access
+
+#### Creating pull requests
+1. create a new branch
+1. push your changes to the branch
+1. create a draft pull request
+1. when you think it is ready, mark it as "ready for review" and invite a reviewer