Added URL to the "pickle exceptions" discussion on Reddit in the comments

celery/tests/test_pickle.py

@@ -1,6 +1,5 @@
 import unittest
 import pickle
-from django.http import Http404
 
 class RegularException(Exception):
     pass
@@ -13,6 +12,8 @@ class ArgOverrideException(Exception):
 
 
 class TestPickle(unittest.TestCase):
+    # See: http://www.reddit.com/r/django/comments/8gdwi/
+    #      celery_distributed_task_queue_for_django/c097hr1
 
     def test_pickle_regular_exception(self):