Add Code Climate badge to README.md

As of this commit, we are getting a clean 4.0 score from Code Climate.
Let's show this off.

Change-Id: I421818691bdc8f721d9f178a775582cc47a28ae4
Reviewed-on: https://gerrit.causes.com/26313
Reviewed-by: Shane da Silva <shane@causes.com>
Tested-by: Joe Lencioni <joe.lencioni@causes.com>