[ci skip] add guide for ActiveModel::Naming

As asked in the #15494, this is the PR for the `ActiveModel::Naming` guide.
For this paragraph, what do you think about :

> The module defines the `model_name` class method which will define a number of accessors using some [`ActiveSupport::Inflector` methods](http://api.rubyonrails.org/classes/ActiveSupport/Inflector.html).

The inflections are only addressed in the "Active Support Core Extensions" guide so a link will make the paragraph a bit more clear for newbies since Active Model guide will be addressed before. What do you think ?