contributing.md

Dear coders and github fork owners: PLEASE MERGE BACK! Regardless of the size of the delta, all fixes are welcome and important.

Some guidelines:

• use Black to ensure you are not introducing new issues by accident
• test your changes (setup.py test)
• write tests for now code if possible
• do not commit '.mo' files - they do not belong here - they can be rebuilt from .po
• propose a merge on github