Issue 35224: Add PEP 572 (assignment expressions) to What's New (#12941)

This is meant as a stub, during the PyCon sprints we can iterate.

Doc/whatsnew/3.8.rst

@@ -67,6 +67,22 @@ Summary -- Release highlights
 New Features
 ============
 
+Assignment expressions
+----------------------
+
+There is new syntax (the "walrus operator", ``:=``) to assign values
+to variables as part of an expression.  Example::
+
+  if (n := len(a)) > 10:
+      print(f"List is too long ({n} elements, expected <= 10)")
+
+See :pep:`572` for a full description.
+
+(Contributed by Emily Morehouse in :issue:`35224`.)
+
+.. TODO: Emily will sprint on docs at PyCon US 2019.
+
+
 Parallel filesystem cache for compiled bytecode files
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