Add README.md.

Author: ilopX

patterns/memento/memento_editor/README.md

@@ -0,0 +1,48 @@
+# 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 a [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/master/patterns/observer/app_observer)]
+- [[SubscriberWidget](https://github.com/RefactoringGuru/design-patterns-dart/tree/master/patterns/observer/subscriber_flutter_widget)]
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/165399085-06835617-8ef1-4e2f-930f-03d730433afb.png)
+
+### Client code:
+```dart
+class MementoEditorApplication {
+  final editor = Editor();
+  final caretaker = Caretaker();
+
+  void createDefaultShapes() {/*...*/}
+
+  void saveState() {
+    final snapshot = editor.backup();
+
+    if (caretaker.isSnapshotExists(snapshot)) {
+      return;
+    }
+
+    final memento = Memento(DateTime.now(), snapshot);
+    caretaker.addMemento(memento);
+    editor.events.notify(MementoCreateEvent());
+  }
+
+  void restoreState(Memento memento) {
+    editor
+      ..unSelect()
+      ..restore(memento.snapshot)
+      ..repaint();
+  }
+}
+```