Add docs badge

Hi there,

I made a docs badge for Elixir projects. Since Elixir declared documentation to be a "first class citizen" and I already had created [Inch](http://trivelop.de/inch) for Ruby projects, I decided to try to implement "inline-docs linting" for Elixir as well.

This is what it would look like: [![Inline docs](http://inch-ci.org/github/undeadlabs/discovery.svg)](http://inch-ci.org/github/undeadlabs/discovery) (click to view status page)

The idea is to show to potential contributors that a lib/package is documented and to inspire people starting with Elixir to follow suit. The badge links to [Inch CI](http://inch-ci.org), a project that tries to raise the visibility of inline-docs to encourage aspiring programmers to document their code.  Inch CI is still relatively new (4 months old), but is also relatively succesful (badges used by 400+ Ruby projects and some Elixir projects including the [Phoenix Webframework](https://github.com/phoenixframework/phoenix) and [Plug](https://github.com/elixir-lang/plug)).

What do you think?