CONTRIBUTORS: Add Gustavo Franco (Google CLA)

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/7235047

CONTRIBUTORS

@@ -165,6 +165,7 @@ Gideon Jan-Wessel Redelinghuys <gjredelinghuys@gmail.com>
 Giles Lean <giles.lean@pobox.com>
 Graham Miller <graham.miller@gmail.com>
 Gustav Paul <gustav.paul@gmail.com>
+Gustavo Franco <gustavorfranco@gmail.com>
 Gustavo Niemeyer <gustavo@niemeyer.net> <n13m3y3r@gmail.com>
 Gwenael Treguier <gwenn.kahz@gmail.com>
 Han-Wen Nienhuys <hanwen@google.com>