Add DEVELOPING.md, take a README pass

[sritchie]

With the typescript changes, test changes etc it's time that we add a DEVELOPING.md.

@cchudzicki , I would love if we could have notes on

• [ ] how to develop with typescript (how to add types, how to convert a file)
• [x] how to run the tests
• [x] how to generate docs

that sort of thing. We might also link in the README to your new react project and the demo project.