CONTRIBUTORS: add David Crawshaw's codereview account

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5339047

CONTRIBUTORS

@@ -85,7 +85,7 @@ Daniel Theophanes <kardianos@gmail.com>
 Dave Cheney <dave@cheney.net>
 Dave Grijalva <dgrijalva@ngmoco.com>
 David Anderson <danderson@google.com>
-David Crawshaw <david.crawshaw@zentus.com>
+David Crawshaw <david.crawshaw@zentus.com> <crawshaw@google.com>
 David Forsythe <dforsythe@gmail.com>
 David G. Andersen <dave.andersen@gmail.com>
 David Jakob Fritz <david.jakob.fritz@gmail.com>