add Factory Method pattern

factory-method/README.md

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+## Factory Method
+
+Define an interface for creating an object, but let subclasses decide which class to instantiate. 
+Factory Method lets a class defer instantiation to subclasses. The pattern has creational purpose
+and applies to classes where deals with relationships through inheritence ie. they are static-fixed 
+at compile time. In contrast to Abstract Factory, Factory Method contain method to produce only one
+type of product.  
+
+### When to use
+
+* a class cant anticipate the class of objects it must create
+* a class wants its subclasses to specify the objects it creates
+* classes delegate responsibility to one of several helper subclasses, and you want to localize the knowledge of which helper subclass is the delegate