Pointing out that dynamic_form plugin must be installed to be able to…

… use error_messages and error_messages_for view helpers.

railties/guides/source/active_record_validations_callbacks.textile

@@ -738,7 +738,20 @@ person.errors.size # => 0
 
 h3. Displaying Validation Errors in the View
 
-Rails provides built-in helpers to display the error messages of your models in your view templates.
+Rails maintains an official plugin that provides helpers to display the error messages of your models in your view templates. You can install it as a plugin or as a Gem.
+
+h4. Installing as a plugin
+<shell>
+$ rails plugin install git://github.com/rails/dynamic_form.git
+</shell>
+
+h4 Installing as a Gem
+Add this line on your Gemfile:
+<ruby>
+gem "dynamic_form"
+</ruby>
+
+Now you will have access to these two methods in your view templates:
 
 h4. +error_messages+ and +error_messages_for+