Adds more criticism to Singleton pattern.

singleton/README.md

@@ -40,6 +40,8 @@ Use the Singleton pattern when
 
 * Violates Single Responsibility Principle (SRP) by controlling their own creation and lifecycle.
 * Encourages using a global shared instance which prevents an object and resources used by this object from being deallocated.     
+* Creates tightly coupled code that is difficult to test.
+* Makes it almost impossible to subclass a Singleton.
 
 ## Credits