add a note about DynamicForm in error_messages_for section [ci skip]

guides/source/i18n.md

@@ -816,7 +816,8 @@ So, for example, instead of the default error message `"can not be blank"` you c
 
 #### Translations for the Active Record `error_messages_for` Helper
 
-If you are using the Active Record `error_messages_for` helper, you will want to add translations for it.
+If you are using the Active Record `error_messages_for` helper, you will want to add
+translations for it.
 
 Rails ships with the following translations:
 
@@ -831,6 +832,9 @@ en:
         body:    "There were problems with the following fields:"
 ```
 
+NOTE: In order to use this helper, you need to install [DynamicForm](https://github.com/joelmoss/dynamic_form)
+gem. You can install it as a gem by adding this line to your Gemfile: `gem 'dynamic_form'`.
+
 ### Overview of Other Built-In Methods that Provide I18n Support
 
 Rails uses fixed strings and other localizations, such as format strings and other format information in a couple of helpers. Here's a brief overview.