[MAJOR] Missing contributing guide

[niksirbi]

@calebweinreb you seem to be the only developer at the moment, but I assume that you would like for the project to grow and to inspire contributions from collaborators, and perhaps also from the wider systems neuro community.

I suggest adding a CONTRIBUTING.md guide to the repo and, at the minimum, stating your policy on external contributions. Are they welcome, and if so, what's the recommended procedure?

[calebweinreb]

I've now added a contribution guide:

https://github.com/calebweinreb/SNUB/blob/main/CONTRIBUTING.md

[niksirbi]

Thanks! I'll close this issue.