add Memento pattern

memento/README.md

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+## Memento
+
+Memento without violating encapsulation, captures and externalizes an object's internal
+state so that the object can be restored to this state later. The pattern has behavioral
+purpose and applies to the objects.
+
+### When to use
+
+* a snapshot of an object's state must be saved so that it can be restored to that state later
+* a direct interface to obtaining the state would expose implementation details and break the object's encapsulation