Add example to to_representation docs (#5682)

Closes #5425 as per #5425 (comment)

Author: carltongibson

docs/api-guide/serializers.md

@@ -1007,6 +1007,14 @@ The signatures for these methods are as follows:
 
 Takes the object instance that requires serialization, and should return a primitive representation. Typically this means returning a structure of built-in Python datatypes. The exact types that can be handled will depend on the render classes you have configured for your API.
 
+May be overridden in order modify the representation style. For example:
+
+    def to_representation(self, instance):
+        """Convert `username` to lowercase."""
+        ret = super().to_representation(instance)
+        ret['username'] = ret['username'].lower()
+        return ret
+
 #### ``.to_internal_value(self, data)``
 
 Takes the unvalidated incoming data as input and should return the validated data that will be made available as `serializer.validated_data`. The return value will also be passed to the `.create()` or `.update()` methods if `.save()` is called on the serializer class.