docs/whatsnew: Mention that 'async' and 'await' will be keywords in 3.7

Doc/whatsnew/3.5.rst

@@ -805,6 +805,14 @@ Changes to Python's build process and to the C API include:
 Deprecated
 ==========
 
+New Keywords
+------------
+
+``async`` and ``await`` are not recommended to be used as variable, class or
+function names.  Introduced by :pep:`492` in Python 3.5, they will become
+proper keywords in Python 3.7.
+
+
 Unsupported Operating Systems
 -----------------------------