added new example for abstract factory pattern

AbstractFactory/AbstractFactory.php

@@ -14,6 +14,7 @@ namespace DesignPatterns;
  * - SQL Factory (types are all strings with SQL, but they vary in detail (tables, fields, etc.))
  * - Zend Framework: Zend_Form::createElement() creates form elements, but you could also call new T
  *   TextElement() instead
+ * - an abstract factory to create various exceptions (e.g. Doctrine2 uses this method)
  *
  */
 abstract class AbstractFactory