README.md in ahoy_email-0.2.4 vs README.md in ahoy_email-0.3.0

@@ -114,10 +114,20 @@
 <a data-skip-utm-params="true" href="...">Break it down</a>
 ```
 
 ### Extra Attributes
 
-Create a migration to add extra attributes to the `ahoy_messages` table and use:
+Create a migration to add extra attributes to the `ahoy_messages` table, for example:
+
+```ruby
+class AddCampaignIdToAhoyMessages < ActiveRecord::Migration
+  def change
+    add_column :ahoy_messages, :campaign_id, :integer
+  end
+end
+```
+
+Then use:
 
 ```ruby
 track extra: {campaign_id: 1}
 ```