docs: Add CONTRIBUTING

CONTRIBUTING.md

+# How to Contribute
+
+- Assign yourself to the issue you want to tackle. Add the DOING label
+- Create a seperate branch for the issue
+- For your commit messages please use the [this](https://dev.to/i5han3/git-commit-message-convention-that-you-can-follow-1709) guideline
+- If you solved the issue you can open a Merge Request and assign somebody other than yourself to approve your changes