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

LGTM=bradfitz, austin
R=austin
CC=bradfitz, golang-codereviews
https://codereview.appspot.com/158330045

wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old