Add README.

Author: ilopX

patterns/mediator/conceptual/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/mediator).
+
+## Diagram:
+![image](https://user-images.githubusercontent.com/8049534/173237874-971dd4e7-2e74-4cac-bcea-77b88255adad.png)
+
+### Client code:
+```dart
+void main() {
+  final component1 = Component1();
+  final component2 = Component2();
+
+  ConcreteMediator(component1, component2);
+
+  component1.doOne();
+  print('');
+  component2.doTwo();
+}
+```
+
+### Output:
+```
+call Component1.doOne()
+ConcreteMediator.notify(event: "doOne")
+ConcreteMediator.reactComponentOne()
+use component2.name = "Two"
+
+call Component2.doTwo()
+ConcreteMediator.notify(event: "doTwo")
+ConcreteMediator.reactComponentTwo()
+use component1.sate = "Cmp1"
+```
+