Add README.

Author: ilopX

patterns/state/manipulator_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).
+
+### Online demo:
+Click on the picture to see the [demo](https://RefactoringGuru.github.io/design-patterns-dart/#/state/manipulator_state).
+
+[![image](https://user-images.githubusercontent.com/8049534/171070341-1decb58f-033b-4eb5-89d4-355aafa6b680.png)](https://refactoringguru.github.io/design-patterns-dart/#/state/manipulator_state)
+
+### Video
+https://user-images.githubusercontent.com/8049534/171499203-1400c3ae-d5cd-4e48-a0b6-0252f4345d19.mp4
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/171740942-659d3ec9-8355-4078-a7d6-b4a338b41187.png)
+
+## Client code:
+### Change FreeState to MoveState:
+```dart
+class FreeState extends ManipulationState {
+  @override
+  void mouseDown(double x, double y) {
+    tryToSelectAndStartMovingShape(x, y);
+  }
+
+  bool tryToSelectAndStartMovingShape(double x, double y) {
+    final selectedShape = context.shapes.findShapeByCoordinates(x, y);
+
+    context.changeState(
+      MoveState(
+        startX: x,
+        startY: y,
+        selectedShape: selectedShape,
+      ),
+    );
+
+    return true;
+  }
+}
+```
+
+### Change MoveState to ResizableState:
+```dart
+class MoveState extends SelectionState {
+  @override
+  void mouseMove(double x, double y) {
+    selectedShape.move(x, y);
+    context.update();
+  }
+
+  @override
+  void mouseUp() {
+    context.changeState(
+      selectedShape.createSelectionState(),
+    );
+  }
+}
+```
+
+### Each shape has its own state manipulator:
+```dart
+class RectangleShape extends BaseShape {
+  @override
+  SelectionState createSelectionState() {
+    return ResizableState(selectedShape: this);
+  }
+}
+
+class CircleShape extends BaseShape {
+  @override
+  SelectionState createSelectionState() {
+    return InnerRadiusState(selectedShape: this);
+  }
+}
+```