Add illustration of state machine design pattern

Author: gjbex

source-code/design-patterns/README.md

@@ -8,9 +8,11 @@ Code to illustrate some design patterns in Python.
    can be used to change the behaviour of objects.
 1. `factory_design_pattern.ipynb`: notebook illustrating how a factory class
    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. 
+1. `state_machine_design_pattern.ipynb`: notebook illustrating how a state
+   machine can be implemented in Python.
+1. `finite-state-parser`: illustration of object-oriented data
+    representation and the state pattern.