add a simple example with a null logger

NullObject/LoggerInterface.php

@@ -0,0 +1,16 @@
+<?php
+
+/*
+ * DesignPatternPHP
+ */
+
+namespace DesignPatterns\NullObject;
+
+/**
+ * LoggerInterface is a contract for logging something
+ */
+interface LoggerInterface
+{
+
+    public function log($str);
+}