[JOSS] Contributing guidelines

[sappelhoff]

(related to openjournals/joss-reviews#3293)

In the docs I found that you "welcome field reports" from Windows users. Other than that however, I did not see documentation on:

• in how far contributions to this project are welcome
• how to do them (a developer setup of the software, project specific issues/PR guidelines, etc.)
• which channel to use to ask for help or contact you: Just use GitHub issues?

I think that although some of these points may have an implicit answer (e.g., users should just ask on GitHub Issues), there is a benefit in spelling this out in a dedicated documentation.

I recommend to either create a CONTRIBUTING.md file at the root of the project, and linking to it from the docs and README, or to prepare a dedicated page in the docs, and link to it from the README (and the docs toc).

[sappelhoff]

PS: The developer notes should also contain information on how to run the complete test suite.

[sappelhoff]

Thanks, I think this has been well addressed.