Docs: Add contributing guidlines

CONTRIBUTING.md

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+# Contributing
+
+Please stick to the following recommendations when making contributions to this repo.
+
+## General
+
+- Each commit should be logically self-contained and shall "compile" without warnings or errors
+- Please split pure refactoring or formatting changes from features
+
+
+## Commit Message Format
+
+A commit message consists of the header line and a bunch of additional lines explaining the changes.
+
+The header line shall use the following base format:
+
+```
+topic: Short description
+```
+
+If the commit is a pure refactoring or code-formatting change, it shall have the prefix `[TIDY]` and bugfixes shall have the prefix `[FIX]`.
+
+Examples:
+
+- `[TIDY] CI: Move common parts to own job`
+- `Style: Make navbar sticky`