CONTRIBUTORS: add Dave Cheney <dave@cheney.net>

R=golang-dev
CC=golang-dev
http://codereview.appspot.com/4257065

CONTRIBUTORS

@@ -6,6 +6,7 @@
 # Please keep the list sorted.
 
 
+Dave Cheney <dave@cheney.net>
 David Symonds <dsymonds@golang.org>
 Ken Thompson <ken@golang.org>
 Mikkel Krautz <mikkel@krautz.dk> <krautz@gmail.com>