docs: add paragraph on transform_fieldname methods

docs/api-guide/serializers.md

@@ -63,6 +63,21 @@ At this point we've translated the model instance into Python native datatypes.
     json
     # '{"email": "leila@example.com", "content": "foo bar", "created": "2012-08-22T16:20:09.822"}'
 
+### Customizing field representation
+
+Sometimes when serializing objects, you may not want to represent everything exactly the way it is in your model.
+
+If you need to customize the serialized value of a particular field, you can do this by creating a `transform_<fieldname>` method. For example if you needed to render some markdown from a text field:
+
+    description = serializers.TextField()
+    description_html = serializers.TextField(source='description', read_only=True)
+
+    def transform_description_html(self, obj, value):
+        from django.contrib.markup.templatetags.markup import markdown
+        return markdown(value)
+
+These methods are essentially the reverse of `validate_<fieldname>` (see *Validation* below.)
+
 ## Deserializing objects
         
 Deserialization is similar.  First we parse a stream into Python native datatypes...