title: Django: add index to meta field
body: <p>I have the following model:</p>
  
  <pre><code>from model_utils.models import TimeStampedModel
  class MyModel(TimeStampedModel):
      ....
      name = models.CharField(max_length=100)
      ....
  </code></pre>
  
  <p>This model is based on Django Utils (<a href="https://django-model-utils.readthedocs.org/en/latest/models.html#timestampedmodel" rel="nofollow">https://django-model-utils.readthedocs.org/en/latest/models.html#timestampedmodel</a>) which adds <code>created</code> and <code>modified</code> fields to my model.</p>
  
  <p>What I need know is way to add a <code>db_index</code> to the modified field of my model. But I can't modify the TimeStampedModel file because it is part of an external dependency.</p>
  
  <p>Do you know any easy solution for this kind of issue?</p>
  
  <p>Thank you.</p>