Add Code Climate badge to README

I found code climate to be useful to track down extra complexity and improve my own projects. Thus I added it to NMatrix (it's free for open source!) and I hope you folks find it useful as well.

You can see the [grades for each class](https://codeclimate.com/github/SciRuby/nmatrix/code) and some of [the issues it found](https://codeclimate.com/github/SciRuby/nmatrix/issues).