CONTRIBUTORS: Add Ryan Barrett (Google CLA)

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6244071

CONTRIBUTORS

@@ -266,6 +266,7 @@ Roger Peppe <rogpeppe@gmail.com>
 Ron Minnich <rminnich@gmail.com>
 Ross Light <rlight2@gmail.com>
 Russ Cox <rsc@golang.org>
+Ryan Barrett <ryanb@google.com>
 Ryan Hitchman <hitchmanr@gmail.com>
 Sam Thorogood <thorogood@google.com> <sam.thorogood@gmail.com>
 Sameer Ajmani <sameer@golang.org> <ajmani@gmail.com>