Add docs badge to README

Hi Christoph,

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/colszowka/simplecov.png)](http://inch-pages.github.io/github/colszowka/simplecov)

The badge links to [Inch Pages](http://inch-pages.github.io), a project that tries to raise the visibility of documentation in Ruby projects. Your status page is http://inch-pages.github.io/github/colszowka/simplecov/

Inch Pages is still in it's infancy, but already used by projects like [Guard](https://github.com/guard/guard), [Haml](https://github.com/haml/haml), [Pry](https://github.com/pry/pry), [RuboCop](https://github.com/bbatsov/rubocop), and [Libnotify](https://github.com/splattael/libnotify).

What do you think?