[ADD] CONTRIBUTING.md for the hacktoberfest

CONTRIBUTING.md

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+# Contributing
+So you're thinking about contributing to our server workspace, great!
+To make sure every contributor is on the same 'page', the CommIT team advises to keep the following guidelines in mind when contributing.
+
+## Branching Strategy
+For branching, we apply the same branching strategy as we use in Koala:
+- `feature/` for new features.
+- `bug/` for bugfixes.
+- `doc/` for documentation.
+- `test/` for testing.
+- `debt/` for refactoring and enhancements.
+
+## Commit Messages
+Please make sure to write descriptive commit messages:
+1. The header should be 72 characters or less.
+2. Write messages in the imperative: "Fix bug X", "Add feature Y", etc.
+3. Reference issue in commit that fixes it: "Fixes #12: Remove bug Z".
+4. Add a body to the message in which you elaborate on the header.
+
+## Thanks!
+Thank you a lot for any PR's!
+