Add docs badge to README

Hi guys,

this patch would add a docs badge to the README to show off inline-documentation to potential contributors: [![Inline docs](http://inch-ci.org/github/elixir-lang/plug.svg?branch=master)](http://inch-ci.org/github/elixir-lang/plug)

The badge links to [Inch CI](http://inch-ci.org), a project that tries to raise the visibility of inline-docs to encourage aspiring Elixir programmers to document their public modules/functions and treat documentation as a first class citizen.

The Elixir support is still new, but [I already convinced Chris and José](https://github.com/phoenixframework/phoenix/pull/404) to give it a try and include it in the Phoenix Framework's README and we are having an active discussion about what can be changed to make this a useful tool for the Elixir community.

What do you think?