Add docs badge to README

I just added the badge you requested to your README: ![Inline docs](http://inch-pages.github.io/github/pboling/gem_bench.png)

I did not know where to put it, since there are no of the usual suspects present in this project, so I put it next to the title.

Your status page for this project is http://inch-pages.github.io/github/pboling/gem_bench