added external design pattern documentation links to structural patterns.

pypatterns/structural/composite.py

@@ -1,6 +1,8 @@
 class Composite(object):
     """
     Composite class as part of the Composite pattern.
+
+    External Composite Pattern documentation: U{https://en.wikipedia.org/wiki/Composite_pattern}
     """
     def __init__(self, interface):
         """