Add comment about rails-controller-testing gem

Author: Josh Justice

lib/generators/rspec/scaffold/templates/controller_spec.rb

@@ -17,6 +17,11 @@
 # is no simpler way to get a handle on the object needed for the example.
 # Message expectations are only used when there is no simpler way to specify
 # that an instance is receiving a specific message.
+#
+# Also compared to earlier versions of this generator, there are no longer any
+# expectations of assigns and templates rendered. These features have been
+# removed from Rails core in Rails 5, but can be added back in via the
+# `rails-controller-testing` gem.
 
 <% module_namespacing do -%>
 RSpec.describe <%= controller_class_name %>Controller, <%= type_metatag(:controller) %> do