Add a hex.pm version badge to the README

The badge is provided by http://shields.io and shows the version of the package on [hex.pm](https://hex.pm). Maybe not super useful, but IMO it polishes the repository look on GitHub and at the same time it gives a quick view of what version to add in a `mix.exs` file.

**Edit** you can see the badge in action [on my fork](https://github.com/whatyouhide/plug/tree/hexpm-version-badge).