Add porting note for issue #20951.
diff --git a/Doc/whatsnew/3.5.rst b/Doc/whatsnew/3.5.rst
index b9ad684..02ff833 100644
--- a/Doc/whatsnew/3.5.rst
+++ b/Doc/whatsnew/3.5.rst
@@ -213,3 +213,7 @@
if it represented midnight in UTC. This behavior was considered obscure and
error-prone and has been removed in Python 3.5. See :issue:`13936` for full
details.
+
+* :meth:`ssl.SSLSocket.send()` now raises either :exc:`ssl.SSLWantReadError`
+ or :exc:`ssl.SSLWantWriteError` on a non-blocking socket if the operation
+ would block. Previously, it would return 0. See :issue:`20951`.