CONTRIBUTORS: add Austin Clements's google.com email (Google CLA)

LGTM=bradfitz, austin
R=austin
CC=bradfitz, golang-codereviews
https://golang.org/cl/158330045

CONTRIBUTORS

@@ -96,7 +96,7 @@ Arvindh Rajesh Tamilmani <art@a-30.net>
 Asim Shankar <asimshankar@gmail.com>
 Ato Araki <ato.araki@gmail.com>
 Aulus Egnatius Varialus <varialus@gmail.com>
-Austin Clements <aclements@csail.mit.edu>
+Austin Clements <austin@google.com> <aclements@csail.mit.edu>
 Balazs Lecz <leczb@google.com>
 Ben Eitzen <eitzenb@golang.org>
 Ben Fried <ben.fried@gmail.com>