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