Add docs badge to README

Hi Konstantin,

this patch adds an alternative docs badge to the README to show off inline-documentation to the casual visitor: [![Inline docs](http://inch-pages.github.io/github/rkh/mustermann.png)](http://inch-pages.github.io/github/rkh/mustermann) 

The badge shows an evaluation of the present inline-docs in your project and 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 Mustermann is http://inch-pages.github.io/github/rkh/mustermann/

Inch Pages is still in it's infancy, but already used by projects like [Guard](https://github.com/guard/guard) and [Pry](https://github.com/pry/pry).

What do you think?

P.S. I wanted to leave it to you to decide whether or not this patch, if accepted, makes the present yardoc badge obsolete.