According to DHH a Rails application does not need too much documentation. (check the post here)

The first, and the “good enough” reason to test your Ruby on Rails application thoroughly is that Ruby is not a compiler but an interpreter, if not tested your application can halt even on a banal syntax error.

RailRoad generates nice diagrams of your models and controllers.

You’ll have to document your app. First you’ll have to describe what it should do (via RSpec), and then how it does (using doc & diagram generator plugins).