title: Add timestamps to an existing table
body: <p>I need to add timestamps (<code>created_at</code> &amp; <code>updated_at</code>) to an existing table. I tried the following code but it didn't work.</p>
  
  <pre><code>class AddTimestampsToUser &lt; ActiveRecord::Migration
      def change_table
          add_timestamps(:users)
      end
  end
  </code></pre>