Nav cleanup

Quick first take at addressing #40 from the Reads Task Team repo:
- clarify that this is a *developer* site
- add a hand-maintained section at the top that links to API documentation
- clearly label the repos as source code
- add a README

(Note: I don't know how to test changes to a GitHub Page, so while I read it carefully, there could be typos -- lmk if anyone has suggestions.)