Add client code & output to README.

Author: ilopX

patterns/abstract_factory/conceptual_gui_factory/README.md

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+# Memento pattern
+Memento is a behavioral design pattern that lets you save and restore the previous state of an 
+object without revealing the details of its implementation.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/memento).
+
+### Online demo:
+Click on the picture to see the [demo](https://RefactoringGuru.github.io/design-patterns-dart/#/memento/flutter_memento_editor).
+
+[![image](https://user-images.githubusercontent.com/8049534/165401175-88bc4593-4624-45b4-8c03-6f1390ed771a.png)](https://refactoringguru.github.io/design-patterns-dart/#/memento/flutter_memento_editor)
+
+### Dependency Patterns
+This complex example includes these implementations:
+- [[AppObserver](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/observer/app_observer)]
+- [[SubscriberWidget](https://github.com/RefactoringGuru/design-patterns-dart/tree/main/patterns/observer/subscriber_flutter_widget)]
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/165758516-1de543f5-666d-4e07-958d-2d8fceb73af9.png)
+
+### Client code:
+```dart
+void main() {
+  final guiFactory = GUIFactory();
+  final app = Application(guiFactory);
+  app.paint();
+}
+
+abstract class GUIFactory {
+  factory GUIFactory() {
+    if (Platform.isMacOS) {
+      return MacOSFactory();
+    } else {
+      return WindowsFactory();
+    }
+  }
+  
+  /*...*/
+}
+```
+
+### Output:
+```
+You have created WindowsButton.
+You have created WindowsCheckbox.
+```