routing guide: documents the CONTROLLER environment variable understo…

…od by the routes task

railties/guides/source/routing.textile

@@ -762,6 +762,12 @@ formatted_users GET  /users.:format  {:controller=>"users", :action=>"index"}
                 POST /users.:format  {:controller=>"users", :action=>"create"}
 </pre>
 
+You may restrict the listing to the routes that map to a particular controller setting the +CONTROLLER+ environment variable:
+
+<shell>
+$ CONTROLLER=users rake routes 
+</shell>
+
 TIP: You'll find that the output from +rake routes+ is much more readable if you widen your terminal window until the output lines don't wrap.
 
 h4. Testing Routes