Add developer docs

[calebsmith]

• Needs a requirements file for test requirements (tox and sphinx)
• Needs some instructions on local setup for developers, especially with regard to the settings file to point to for tests

[calebsmith]

The commit referenced above is a nice start, and I think it is a sufficient amount of developer docs for the 0.3.4 release, but I'd like to add some more details about things, so I'm leaving this ticket open.