88 - Added undo example for command pattern

Behavioral/Command/UndoableCommandInterface.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace DesignPatterns\Behavioral\Command;
+
+/**
+ * Interface UndoableCommandInterface.
+ */
+interface UndoableCommandInterface
+{
+    /**
+     * This method is used to undo change made by command execution
+     * The Receiver goes in the constructor.
+     */
+    public function undo();
+}