add backwards compatibility comment

pep-0487.txt

@@ -350,6 +350,20 @@ if keyword arguments are given::
 
 Defining both ``__init__`` and ``__new__`` continues to work fine.
 
+About the only thing that stops working is passing the arguments of 
+``type.__new__`` as keyword arguments::
+
+    class MyMeta(type):
+        def __new__(cls, name, bases, namespace):
+            return super().__new__(cls, name=name, bases=bases,
+                                   dict=namespace)
+                                   
+    class MyClass(metaclass=MyMeta):
+        pass
+       
+This will now raise ``TypeError``, but this is weird code, and easy
+to fix even if someone used this feature.
+
 
 Rejected Design Options
 =======================