add State pattern

state/README.md

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+## State
+
+The pattern allows an object to alter its behavior when its internal state changes.
+The object will appear to change its class. It has behavioral purpose and applies 
+to the objects.
+
+### When to use
+
+* when an object's behavior depends on its state, and it must change its behavior at run-time depending on that state
+* operations have large, multipart conditional statements that depend on the object's state