document the shortcut to the root route helper [ci skip]

This was implemented in 2ee4dd85

actionpack/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+* Allows the route helper `root` to take a string argument. For example, `root 'pages#main'`. *bcardarella*
+
 * Forms of persisted records use always PATCH (via the `_method` hack). *fxn*
 
 * For resources, both PATCH and PUT are routed to the `update` action. *fxn*

railties/guides/source/routing.textile

@@ -622,6 +622,7 @@ You can specify what Rails should route +"/"+ to with the +root+ method:
 
 <ruby>
 root :to => 'pages#main'
+root 'pages#main' # shortcut for the above
 </ruby>
 
 You should put the +root+ route at the top of the file, because it is the most popular route and should be matched first. You also need to delete the +public/index.html+ file for the root route to take effect.