[JOSS] add contribution guidelines

[Fabian-Robert Stöter]

The JOSS review criteria asks for "clear guidelines for third-parties wishing to"

• Contribute to the software
• Report issues or problems with the software
• Seek support

ideally, add this to a contributing.md

This issue is part of my JOSS review openjournals/joss-reviews#2989

[hadware]

Thanks for that suggestion!

I added a CONTRIBUTING file to the repo, i hope it fits the bill. Do tell me if something isn't right.

[Fabian-Robert Stöter]

looks great!