Add open issues section.

pep-0353.txt

@@ -229,6 +229,18 @@ containers doesn't change, e.g.
   type on most 64-bit systems (except Win64), so
   the compiler inserts padding before it as well.
 
+Open Issues
+===========
+
+* Marc-Andre Lemburg commented that complete backwards
+  compatibility with existing source code should be
+  preserved. In particular, functions that have
+  Py_ssize_t* output arguments should continue to run
+  correctly even if the callers pass int*.
+
+  It is not clear what strategy could be used to implement
+  that requirement.
+
 
 Copyright
 =========