Update _data/README.md

Now reflects the current state of the world, where 18F/data-private is the one authoritative source repo for team data and the _data/import-public.rb script is used to populate _data/public.
Update README.md

Tightens up the language and structure, and references the blog posts on "The 18F Hub" and "Snippets".
Mention schema and public Hub propagation

Also updates section headers and tightens up the language in a couple other places.
Update deploy/README; internal vs. public Hub
Remove vestigial news index plugin info

A news index plugin may be added one day, but best to clear the data now to avoid confusion.
Merge pull request #27 from 18F/update-docs

Update docs