Create FAQ documentation (GH-2247)

This commit creates a Frequently Asked Questions document for our users
to read. Hopefully they actually read it too. Items included are:
Black's non-API, AST safety, style stability, file discovery, Flake8
disagreements and Python 2 support. Hopefully I've got the answers
down in general.

Commit history before merge:

* Create FAQ
* Address feedback
* Move to single markdown file
* Minor wording improvements
* Add changelog entry