Docs: custom authentication now more obvious

I was having a bit of difficulty figuring out where exactly to implement my own authentication method, until I found http://dan.doezema.com/2012/02/how-to-implement-a-single-user-model-with-rails-activeadmin-and-devise/ and realized I should be implementing them in ApplicationController.

Hoping this helps fellow ActiveAdmin newbies.
Merge pull request #3119 from AlexeyMK/master

Docs: custom authentication now more obvious