First copy edit on docs and small improvements to navigation

I did a once-over on the core pages and made small copy edits as well as usability tweaks by inserting links in places I felt they were lacking.

The two main changes were:
1. Add links to the README.rst file to help users who are interested in the points of differentiation find them (b243f1c)
1. Add links between API and Usage pages that help users see the 'real world' examples and switch to the API for more detail as well as vice versa. (9279d51) (51415f6)

Next steps are to go through the Usage and API pages with a copy edit and try doing each of the examples to make sure that they read clearly.