Add README.

Author: ilopX

patterns/state/three_state/README.md

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+# State Pattern
+State is a behavioral design pattern that lets an object alter its behavior when its internal state 
+changes. It appears as if the object changed its class.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/state).
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/172000870-a96c6dd2-5b6a-4a64-b3a9-4edb75dd8076.png)
+
+### Client code:
+```dart
+void main() {
+  final switcher = Switcher(
+    initState: One(),
+  );
+
+  switcher.call(); // call(1): One
+  switcher.call(); // call(2): Two
+  switcher.call(); // call(3): Three
+  switcher.call(); // call:(4) One
+}
+```