Added note about feature requests and rails-core list.

guides/source/contributing_to_ruby_on_rails.textile

@@ -34,6 +34,8 @@ h4. What about Feature Requests?
 
 Please don't put "feature request" items into GitHub Issues. If there's a new feature that you want to see added to Ruby on Rails, you'll need to write the code yourself - or convince someone else to partner with you to write the code. Later in this guide you'll find detailed instructions for proposing a patch to Ruby on Rails. If you enter a wishlist item in GitHub Issues with no code, you can expect it to be marked "invalid" as soon as it's reviewed.
 
+If you'd like feedback on an idea for a feature before doing the work for make a patch, please send an email to the "rails-core mailing list":https://groups.google.com/forum/?fromgroups#!forum/rubyonrails-core. You might get no response, which means that everyone is indifferent. You might find someone who's also interested in building that feature. You might get a "This won't be accepted." But it's the proper place to discuss new ideas. GitHub Issues are not a particularly good venue for the sometimes long and involved discussions new features require.
+
 h3. Running the Test Suite
 
 To move on from submitting bugs to helping resolve existing issues or contributing your own code to Ruby on Rails, you _must_ be able to run its test suite. In this section of the guide you'll learn how to set up the tests on your own computer.