added memento example

src/learn/simple_memento/Main.java

@@ -0,0 +1,19 @@
+package learn.simple_memento;
+
+public class Main {
+    public static void main(String[] args) {
+        Originator originator = new Originator();
+        CareTaker careTaker = new CareTaker();
+
+        originator.setState("blue");
+        originator.setState("red");
+        careTaker.save(originator.createMemento());
+        originator.setState("yellow");
+        careTaker.save(originator.createMemento());
+        originator.setState("white");
+
+        careTaker.revert(originator);
+        careTaker.revert(originator);
+        careTaker.revert(originator);
+    }
+}