Community Contribution Guidelines for JOSS submission

Hi @dwhitena

In finishing up my [review of your JOSS submission](https://github.com/openjournals/joss-reviews/issues/508), I have not been able to find any guidelines for community contributions in the repo. [JOSS says:](https://joss.theoj.org/about#reviewer_guidelines)

> #### Community guidelines
> There should be clear guidelines for third-parties wishing to:
> * Contribute to the software
> * Report issues or problems with the software
> * Seek support

Please let me know if I've missed this information someplace, otherwise please create or adopt a set of guidelines meeting the above requirements and add them in a prominent location in the repo.

Thanks!