title: difference in rails migration with and without :index
body: <p>Given the following two lines </p>
  
  <pre><code>rails g migration add_user_id_to_posts user_id:integer:index
  </code></pre>
  
  <p>and</p>
  
  <pre><code>rails g migration add_user_id_to_posts user_id:integer
  </code></pre>
  
  <p>What is the difference? I can't seem to grasp the concept of <strong>:index</strong>. </p>