Update README

After all this wonderful refactoring, the README was completely lacking
and out-of-date. Update it to include most of the new goodies included
with Overcommit.

Change-Id: Ida62192ad2c8cd6292c527368a3bd8ff6bfe2bb9
Reviewed-on: http://gerrit.causes.com/35623
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Shane da Silva <shane@causes.com>