Add note about discrepancies between this PEP and the new sets module.

pep-0218.txt

@@ -9,6 +9,16 @@ Created: 31-Jul-2000
 Post-History: 
 
 
+Cautionary note
+
+    Much of this PEP has been implemented in the new "sets" module,
+    added to Python 2.3.  The sets module differs in a number of
+    aspects from this PEP; in particular its approach to mutability is
+    different.  It is my hope that this PEP's author will update his
+    PEP to match the sets module more closely.  [note added by Guido
+    van Rossum]
+
+
 Introduction
 
     This PEP proposes adding a Set module to the standard Python