Add what's new entry for r73236.
diff --git a/Doc/whatsnew/3.1.rst b/Doc/whatsnew/3.1.rst
index 1891038..e560bcd 100644
--- a/Doc/whatsnew/3.1.rst
+++ b/Doc/whatsnew/3.1.rst
@@ -384,6 +384,24 @@
(Contributed by Brett Cannon.)
+* :mod:`pickle` is now more compatible with Python 2.x when using a
+ 2.x-compatible protocol (that is, protocol 2 or lower), through translation
+ of some standard library module names to or from their Python 2.x
+ equivalents.
+
+ This means that more (protocol 2 or lower) pickles produced by Python 3.1
+ will be reusable by Python 2.x, and vice-versa. Standard set objects are
+ an example of this improvement.
+
+ This has the (unfortunate but unavoidable) side effect that some
+ protocol 2 pickles produced by Python 3.1 won't be readable with
+ Python 3.0. The latest pickle protocol, protocol 3, should be used when
+ migrating data between Python 3.x implementations, as it doesn't attempt
+ to remain compatible with Python 2.x.
+
+ (Contributed by Alexandre Vassalotti and Antoine Pitrou, :issue:`6137`.)
+
+
Optimizations
=============