title: How to set initial value for read only field
body: <p>I have a django application which is using <code>django.contrib.admin</code> for administrative tasks.</p>
  
  <p>For one model I now need to add a field which indicates which part of the code each row was created from. I am using <code>readonly_fields</code> to prevent this value from being changed through the administration interface.</p>
  
  <p>A default value in this field will tell me that the row was either</p>
  
  <ul>
  <li>created before the field was introduced</li>
  <li>created by code which has not been updated to set the field</li>
  <li>created through the administration interface</li>
  </ul>
  
  <p>But I need better granularity than that. In particular I want to be able to distinguish between a row created by code which doesn't know about the field, and a row created through the administration interface.</p>
  
  <p>Is there some way my <code>ModelAdmin</code> class can specify an initial value for a field mentioned in <code>readonly_fields</code>?</p>