Adding a CONTRIBUTING?

[DanielLenz]

Hi, I have the pleasure to review deepCR for your JOSS submission.

I'll likely have a couple of smaller comments and questions during the next few days, and will raise separate issues to keep your github issues clean.

I really appreciate your paragraph on contributing in the README; do you think it's worth considering to expand this to a full CONTRIBUTING.md? I think it would add value, is easier to find, and gives you the opportunity to provide more detailed guidelines and information on how to contribute.