Add config.ru content to getting started

I think it's important to have this in the docs. I wanted to have a config.ru and didn't want to look through the source code to find it. 

Also, it seems like running `middleman init .` is a destructive process.. Oh well!

First used Middleman about 2 years ago for my first backbone project. I'm so stoked to see how far this has come! Great work. Thanks!