Add contribution guide

CONTRIBUTING.md

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+## Contributing code the right way 
+
+*or: how I learned to post merge requests without crying myself to sleep*
+
+1. Post your idea on the forums, ask to be given developer privileges. Preferably post diffs if you got them. Don't forget to mention your gitlab username. I think it's now possible to request project access but not sure -fox.
+2. When you have developer privileges you can make branches, check them out, work on them locally, push them back and post merge requests.
+3. Usual stuff applies: don't post unrelated issues in one merge request, try to not screw up indenting (tt-rss mostly uses tab characters), try to respect surrounding formatting style, etc.
+4. That's it.