add a TODO note for the inheritance example

src/classes-and-objects.pod

@@ -362,6 +362,9 @@ for a method to dispatch.  This happens both for standard methods defined via
 the method keyword and for methods generated through other means such as 
 attribute accessors.
 
+TODO: the example here is rather bad, and needs to be replaced (or much
+improved). See L<https://github.com/perl6/book/issues/58> for discussion.
+
 =begin programlisting
 
     class Employee {