add comments about null logger

NullObject/NullLogger.php

@@ -26,13 +26,16 @@ namespace DesignPatterns\NullObject;
  * 
  * Performance concerns : ok there is a call for nothing but we spare an "if is_null"
  * I didn't run a benchmark but I think it's equivalent.
+ * 
+ * Key feature : of course this logger MUST implement the same interface (or abstract)
+ * like the other loggers.
  */
 class NullLogger implements LoggerInterface
 {
 
     public function log($str)
     {
-        
+        // do nothing
     }
 
 }