title: Usage of .to_representation() and .to_internal_value in django-rest-framework?
body: <p>What do <code>.to_representation()</code> and <code>.to_internal_value</code> do in <code>serializers</code>?<br>
  If I pass data to a serializer, is the data thrown <code>to_representation()</code> first?<br>
  What's the usage of these two?</p>