Add notes for future selves

Once RubyGems 2.5.0 is required, then the duplicated files can be
removed, and symlinks can be used instead.

[ci skip]

Author: maclover7

actioncable/lib/rails/generators/channel/channel_generator.rb

@@ -24,6 +24,7 @@ def file_name
           @_file_name ||= super.gsub(/\_channel/i, '')
         end
 
+        # FIXME: Change these files to symlinks once RubyGems 2.5.0 is required.
         def generate_application_cable_files
           return if self.behavior != :invoke
 

activerecord/lib/rails/generators/active_record/model/model_generator.rb

@@ -39,6 +39,7 @@ def attributes_with_index
           attributes.select { |a| !a.reference? && a.has_index? }
         end
 
+        # FIXME: Change this file to a symlink once RubyGems 2.5.0 is required.
         def generate_application_record
           if self.behavior == :invoke && !File.exist?('app/models/application_record.rb')
             template 'application_record.rb', 'app/models/application_record.rb'