title: Indexing Django ArrayField or not?
body: <p>Using Django 1.8 on a Postgres 9.6.
  One of my model has an ArrayField:</p>
  
  <pre><code>packages = ArrayField( models.CharField( max_length = 200 ), blank = True )
  </code></pre>
  
  <p>First question is should I add index to it ?
  Documentation says</p>
  
  <blockquote>
    <p>At present using db_index will create a btree index. This does not
    offer particularly significant help to querying. A more useful index
    is a GIN index, which you should create using a RunSQL operation.</p>
  </blockquote>
  
  <p>But it's not clear what I should do with that.</p>
  
  <p>If yes, second question, is where to place "db_index = True" ? 
  Like so?:</p>
  
  <pre><code>packages = ArrayField( models.CharField( db_index = True, max_length = 200 ), blank = True )
  </code></pre>