Add all 23 GoF design pattern implementations

01-creational/factory-method/SlackSender.java

@@ -0,0 +1,9 @@
+package factorymethod;
+
+// New Concrete Creator — knows to create a SlackNotification
+public class SlackSender extends NotificationSender {
+    @Override
+    protected Notification createNotification() {
+        return new SlackNotification();
+    }
+}