Update docs

Now that the Hub has launched, it's time to make sure the READMEs in particular are clear and comprehensive. This PR is just a first pass; this'll be a work in progress for sure. If you can think of a clearer way to express any of this information, feel free to edit a file directly on this [update-docs](https://github.com/18F/hub/tree/update-docs) branch and commit it so it appears in this PR. As with docs in the Hub itself, you can use the [GitHub in-browser editing interface](https://help.github.com/articles/github-flow-in-the-browser/).

@afeld @meiqimichelle @leahbannon @kaitlin 

@konklone @gboone @elainekamlley and @jackiekazil may be interested in deploy/README.md and the private vs. public aspects of data/README.md