Contribution guidelines

[nilshg]

JOSS review comment (issue: JuliaCon/proceedings-review#177)

The JOSS asks for:

Community guidelines: Are there clear guidelines for third parties wishing to 1) Contribute to the software 2) Report issues or problems with the software 3) Seek support

I think 2) is probably fine with the package being hosted on GitHub and having the standard GitHub issue tracker, but 1) and 3) I think need a sentence each in the Readme.