Add README to "Memento" conceptual example.

Author: ilopX

patterns/memento/conceptual/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).
+
+## Conceptual Editor example 
+This example uses the Memento pattern alongside the Command pattern for storing snapshots of the 
+complex text editor’s state and restoring an earlier state from these snapshots when needed.
+
+More detailed explanation on [RefactoringGuru](https://refactoring.guru/design-patterns/memento?#pseudocode).
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/151352367-c97db094-fc87-4eb2-9210-581914c57ced.png)
+
+### Client code:
+```dart
+void main() {
+  final editor = Editor('New Document');
+  final firstState = Command.makeBackup(editor);
+  editor.text += ' add text';
+  final secondState = Command.makeBackup(editor);
+
+  print('Current state: "${editor.text}"');
+
+  firstState.undo();
+  print('First state: "${editor.text}"');
+
+  secondState.undo();
+  print('Second state: "${editor.text}"');
+}
+```
+
+**Output:**
+```
+Current state: "New Document add text"
+First state: "New Document"
+Second state: "New Document add text"
+```