Add Rails section and rule around use of render and return

On https://github.com/airbnb/airbnb/pull/2655 , @clizzin suggested that in cases where `render` and `return` are both used in a statement in ActiveRecord, they should be on different lines. I personally prefer the `render :text => 'hello' and return` way for brevity and readability. I brought the discussion here. What do people think? Yay or nay?