Add docs badge to README

Hi there,

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/redis/redis-rb.png)](http://inch-pages.github.io/github/redis/redis-rb)

The badge 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 the Redis client is http://inch-pages.github.io/github/redis/redis-rb/

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?