title: Django DateTimeField with auto_now_add asks for default
body: <p>I have this field in my model <code>created_at = models.DateTimeField( auto_now_add = True )</code></p>
  
  <p>When I try to make migrations I get an error:</p>
  
  <pre><code>You are trying to add the field 'created_at' with 'auto_now_add=True' to user wi
  thout a default; the database needs something to populate existing rows.
  
   1) Provide a one-off default now (will be set on all existing rows)
   2) Quit, and let me add a default in models.py
  </code></pre>
  
  <p>I tried to set the default value but it says <code>default</code> and <code>auto_now_add</code> are mutually exclusive. Of course I could just use default without <code>auto_now_add</code> but I want to know why this error pops up. Did I miss something?</p>