Libraries document

Some thoughts copied from srittau/type-stub-pep#94:

• Convert to restructed text to fit with the rest of the typing docs (and Python docs in general).
• Update a few recommended practices like using list instead of List.
• Add a link to the existing stubs document.

In the long run, I think this would make a good basis for a "How to type annotate a library" document. It makes sense to me to split out some chapters into separate documents, possibly merging them with existing or new texts. For example, the best practices section could be merged with some recommendations from the stubs document to form a new general "best practices" document (see also python/typing#851).