JOSS review: add community guidelines for sub-projects

[hagenw]

I totally agree with the porposal in #2 (comment) to include the other sub-projects as they are an important part of open-unmix.

In order to fulfill the community guidelines requirements asked for by JOSS, it would be nice if you could add the solution of #6 also to all the sub-projects as they are missing a CONTRIBUTING.md, but they would clearly benefit of having one as well.

[faroit]

Hi, we added this to norbert, museval and musdb. I decided to not do a issue templates for them since we don't except much issues there, so we can probably manage without.

I didn't update the tags since I believe it makes sense that they match to the zenodo release.

[hagenw]

Cool, thank you, this is totally fine with me.