Add notes for porting issues related to pickles.

Doc/whatsnew/3.1.rst

@@ -541,3 +541,8 @@ that may require changes to your code:
     Got:
         2.718281828459045
     **********************************************************************
+
+* The automatic name remapping in the pickle module for protocol 2 or lower can
+  make Python 3.1 pickles unreadable in Python 3.0.  One solution is to use
+  protocol 3.  Another solution is to set the *fix_imports* option to **False**.
+  See the discussion above for more details.