Add comment to clarify that resque/server must be required at the top of the Rails routes file. If you don't do this, you get an uninitialized constant error.

Author: subelsky

README.markdown

@@ -475,7 +475,7 @@ HTTP basic auth).
 
 ### Rails 3
 
-You can also easily mount Resque on a subpath in your existing Rails 3 app by adding this to your `routes.rb`:
+You can also mount Resque on a subpath in your existing Rails 3 app by adding `require resque/server` to the top of your routes file or in an initializer then adding this to `routes.rb`:
 
 ``` ruby
 mount Resque::Server.new, :at => "/resque"