Update API docs, move back to README

It's better here than in github pages.

- Document the `now` endpoints
- Docuemnt `tide_level` parameter of `/predictions/tide-windows/`
- Document `interval` parameter of `/predictions/tide-levels/`
- Add design plans for future endpoints.

API docs update:
https://www.pivotaltracker.com/story/show/74686914 [Delivers #74686914]

Design work:
https://www.pivotaltracker.com/story/show/75795976
[#75795976]