Adds docs on how to add a precompile with Rotor

I'm not entirely sure this is the best place for this documentation, but I do believe it would help a lot of people starting with elixir and/or phoenix that like to use CoffeScript or Sass, to be able to easily integrate those in their workflow.

Further information on how to use Rotor on development and production will be coming along on Rotor's on repo, I believe.