title: read_only field in django rest serializer with unique_together contraint
body: <p>I wanted to make email field unique in default django User model. So i made it <code>unique_together=[('email',)]</code> . Now in serializer I want it to be a read_only field. 
  But Django Rest Framework 3.0 docs says:</p>
  
  <blockquote>
    <p>There is a special-case where a read-only field is part of a
    unique_together constraint at the model level. In this case the field
    is required by the serializer class in order to validate the
    constraint, but should also not be editable by the user.</p>
    
    <p>The right way to deal with this is to specify the field explicitly on
    the serializer, providing both the read_only=True and default=…
    keyword arguments.</p>
    
    <blockquote>
      <p>One example of this is a read-only relation to the currently
      authenticated User which is unique_together with another identifier.
      In this case you would declare the user field like so:</p>
    </blockquote>
    
    <p>user = serializers.PrimaryKeyRelatedField(read_only=True,
    default=serializers.CurrentUserDefault())</p>
  </blockquote>
  
  <p>serializers.CurrentUserDefault() represents the current user.I want to set default as user's email . Isn't <code>serializers.CurrentUserDefault()</code> equivalent to <code>request.user</code> . <code>serializers.CurrentUserDefault().email</code> is giving error <code>'CurrentUserDefault' object has no attribute 'email'</code> How to set email default as user's email ?</p>