Add docs badge to README

Hi Erik,

this patch adds a docs badge to the README to show off inline-documentation to the casual visitor: [![Inline docs](http://inch-pages.github.io/github/sferik/rails_admin.png)](http://inch-pages.github.io/github/sferik/rails_admin)

It links to [Inch Pages](http://inch-pages.github.io), a project that tries to raise the visibility of documentation in Ruby projects. The status page for SimpleForm is http://inch-pages.github.io/github/sferik/rails_admin/

Inch Pages is still in it's infancy, but already used by projects like [Reek](https://github.com/troessner/reek) and [libnotify](https://github.com/splattael/libnotify).

What do you think? :man: