Add gem version badge to README

Also prettied-up the documentation by adding some backticks so fragments
are rendered as code.

Change-Id: I9ba8dd05e0cfa2f75a18df7a65f789ffc7c6e6e5
Reviewed-on: https://gerrit.causes.com/30889
Reviewed-by: Adam Olsen <adam.olsen@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Tested-by: Shane da Silva <shane@causes.com>