Add gem 'arel' from git commented out as an example of how to Bundle egde Rails with Arel edge

[#5723 state:committed]

railties/lib/rails/generators/rails/app/templates/Gemfile

@@ -11,6 +11,7 @@ gem 'rails', '<%= Rails::VERSION::STRING %>'
 
 # Bundle edge Rails instead:
 # gem 'rails', :git => 'git://github.com/rails/rails.git'
+# gem 'arel',  :git => 'git://github.com/rails/arel.git'
 <%- end -%>
 
 <% unless options[:skip_active_record] -%>