Added missing imports to serializer code snippet. Thanks Howard Glynn…

… for reporting.

docs/serialization.rst

@@ -68,9 +68,10 @@ To tweak a format, simply override it's ``to_<format>`` & ``from_<format>``
 methods. So adding the server time to all output might look like so::
 
     import time
+    from django.utils import simplejson
+    from django.core.serializers import json
     from tastypie.serializers import Serializer
 
-
     class CustomJSONSerializer(Serializer):
         def to_json(self, data, options=None):
             options = options or {}