Documentation: Dev Setup

[jrrickerson]

Since this is a learning project, we could use a very thorough A-Z document on how to clone the repo, set up virtualenv, install dependencies, etc, all the way to the point of running the app locally, for anyone who wants to contribute. Linking to existing documents (Github guides, etc) is fine for specific areas of the guide, but it should cover everything a new contributor might need to know.

Ideally let's put this in a CONTRIBUTING markdown file and link to it from the primary README.