Add docs badge to README

Hi there,

this adds the badge you requested. 

You will notice it is not from the Inch Pages project, but the not-yet-loudly-announced Inch CI project, which allows you to refresh the badge yourself [on the page](http://inch-ci.org/github/damien/emigrate) (you can even [configure a webhook to refresh](http://inch-ci.org/howto/webhook) auto-magically).

Let me know what you think! :man: