Installation docs incomplete

First, great project!

Some feedback though. It would ease the unnecessary learning curve for folks who come across this project and are like "what is phoenix? what is mix? how do I get this thing setup?", etc.

Adding some basics to the docs around how to actually use this thing on a computer, such apt-get install erlang elixir, mix local hex, .... would be very helpful to know ahead of time for initial setup.