title: make the user in a model default to the current user
body: <p>I have the following:</p>
  
  <pre><code>from django.contrib.auth.models import User
  
  class ClientDetails(models.Model):
      created_by = models.ForeignKey(User)
      ...
  </code></pre>
  
  <p>How do I make <code>created_by</code> default to the currently logged in user?</p>
  
  <p><em>(I want to do this so I can hide it in the admin view mainly but also because when I save an instance I don't want to be filling it every time)</em></p>