[ci skip] Add `Active Model` to a list of independently used modules.

README.md

@@ -34,7 +34,7 @@ Ruby code (ERB files). Views are typically rendered to generate a controller res
 or to generate the body of an email. In Rails, View generation is handled by Action View.
 You can read more about Action View in its [README](actionview/README.rdoc).
 
-Active Record, Action Pack, and Action View can each be used independently outside Rails.
+Active Record, Active Model, Action Pack, and Action View can each be used independently outside Rails.
 In addition to them, Rails also comes with Action Mailer ([README](actionmailer/README.rdoc)), a library
 to generate and send emails; Active Job ([README](activejob/README.md)), a
 framework for declaring jobs and making them run on a variety of queueing