CONTRIBUTORS: add Paul Borman (Google CLA)

R=borman, bradfitz
CC=golang-dev
https://golang.org/cl/4654079

CONTRIBUTORS

@@ -172,6 +172,7 @@ Padraig Kitterick <padraigkitterick@gmail.com>
 Paolo Giarrusso <p.giarrusso@gmail.com>
 Pascal S. de Kloe <pascal@quies.net>
 Patrick Gavlin <pgavlin@gmail.com>
+Paul Borman <borman@google.com>
 Petar Maymounkov <petarm@gmail.com>
 Peter Froehlich <peter.hans.froehlich@gmail.com>
 Peter McKenzie <petermck@google.com>