Add illustration of iterator pattern

Author: gjbex

source-code/design-patters/README.md

@@ -10,5 +10,7 @@ Code to illustrate some design patterns in Python.
    can be used to conveniently construct many objects with the same properties.
 1. `finite-state-parser`: illustration of object-oriented data
     representation and the state pattern.
+1. `iterator_design_pattern.ipynb`: illustration of the iterator design pattern
+   (as well as the factory pattern).
 1. `strategy_design_pattern.ipynb`: notebook illustrating how a strategy
    design pattern can simplify simulations.