add krautz to CONTRIBUTORS for goprotobuf

R=dsymonds
CC=golang-dev, mkrautz
http://codereview.appspot.com/3308042

CONTRIBUTORS

@@ -8,4 +8,5 @@
 
 David Symonds <dsymonds@golang.org>
 Ken Thompson <ken@golang.org>
+Mikkel Krautz <mikkel@krautz.dk> <krautz@gmail.com>
 Rob Pike <r@golang.org>