Add implementation information.

pep-0252.txt

@@ -393,7 +393,17 @@ Warnings and Errors
 
 Implementation
 
-    XXX
+    A partial implementation of this PEP is available from CVS as a
+    branch named "descr-branch".  To experiment with this
+    implementation, proceed to check out Python from CVS according to
+    the instructions at http://sourceforge.net/cvs/?group_id=5470 but
+    add the arguments "-r descr-branch" to the cvs checkout command.
+    (You can also start with an existing checkout and do "cvs update
+    -r descr-branch".)  For some examples of the features described
+    here, see the file Lib/test/test_descr.py.
+
+    Note: the code in this branch goes beyond this PEP; it is also
+    on the way to implementing PEP 253 (Subtyping Built-in Types).
 
 References