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

webfd/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh