Add Code Climate badge to README

Hello,

Someone (maybe you) added Octopress to Code Climate a few months back, and I thought you might want to know.

We actually use Octopress for our blog, so it's nice to have you represented on our site. You can find your quality metrics here:

https://codeclimate.com/github/imathis/octopress

In case you want this information to be available to contributors, this PR just adds a dynamic badge to the README (similar to Travis) that displays the code quality GPA.

If you have any questions, let me know.

I hope you find Code Climate useful.

Cheers,
Noah