added external design pattern documentation links to behavioural patterns.

pypatterns/behavioral/null.py

@@ -1,6 +1,8 @@
 class Null(object):
     """
     A Null object class as part of the Null object design pattern.
+
+    External Null Object Pattern documentation: U{https://en.wikipedia.org/wiki/Null_Object_pattern}
     """
     def __init__(self, *args, **kwargs):
         """