CONTRIBUTORS: add Matt Jones (Google CLA)

R=golang-dev, r2
CC=golang-dev, mrjones
https://golang.org/cl/4348048

CONTRIBUTORS

@@ -135,6 +135,7 @@ Mark Zavislak <zavislak@google.com>
 Markus Duft <markus.duft@salomon.at>
 Martin Neubauer <m.ne@gmx.net>
 Mathieu Lonjaret <mathieu.lonjaret@gmail.com>
+Matt Jones <mrjones@google.com>
 Maxim Ushakov <ushakov@google.com>
 Micah Stetson <micah.stetson@gmail.com>
 Michael Elkins <michael.elkins@gmail.com>