Add docs badge to README

I just added the badge you requested to your README: ![Inline docs](http://inch-pages.github.io/github/danijoo/Sightstone.png) (although I added the SVG version, since most of your existing badges are in that format)

Your status page for this project is http://inch-pages.github.io/github/danijoo/Sightstone

Thanks for giving this a shot!