added external design pattern documentation links to structural patterns.

pypatterns/structural/flyweight.py

@@ -1,6 +1,8 @@
 class FlyweightMeta(type):
     """
     Flyweight meta class as part of the Flyweight design pattern.
+
+    External Flyweight Pattern documentation: U{https://en.wikipedia.org/wiki/Flyweight_pattern}
     """
     def __new__(mcs, name, bases, attrs):
         """