Added a .parent attribute to the Serializer object for documentation

purposes

djangorestframework/serializer.py

@@ -97,6 +97,11 @@ class Serializer(object):
     The maximum depth to serialize to, or `None`.
     """
     
+    parent = None
+    """
+    A reference to the root serializer when descending down into fields.
+    """
+
     def __init__(self, depth=None, stack=[], **kwargs):
         if depth is not None:
             self.depth = depth