Add Action Cable to README.md of Rails

[ci skip] Link to actioncable/README.md for more details

README.md

@@ -38,7 +38,9 @@ Active Record, Active Model, Action Pack, and Action View can each be used indep
 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
-backends; and Active Support ([README](activesupport/README.rdoc)), a collection
+backends; Action Cable ([README](actioncable/README.md)), a framework to
+integrate WebSockets with a Rails application;
+and Active Support ([README](activesupport/README.rdoc)), a collection
 of utility classes and standard library extensions that are useful for Rails,
 and may also be used independently outside Rails.