Provide essential developer/contributing documentation

[sadielbartholomew]

We should flesh out at least the basics of contributing and developer guidelines, since at present we only have an essential 'contributions welcome' placeholder page. At the least I would like information on the following:

• how to run the test suite (python -m pytest in the tests directory is what I've been going with, but there is no guidance at al in this respect and it would be best to be explicit about the command intended to be used);
• code and docsting formatting requirements;
• how the docs are built.