CONTRIBUTORS: add Jonathan Nieder (Google CLA)

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7842043

CONTRIBUTORS

@@ -227,6 +227,7 @@ Jonathan Feinberg <feinberg@google.com>
 Jonathan Gold <jgold.bg@gmail.com>
 Jonathan Hseu <jhseu@google.com>
 Jonathan Mark <jhmark@xenops.com> <jhmark000@gmail.com>
+Jonathan Nieder <jrn@google.com>
 Jonathan Pittman <jmpittman@google.com> <jonathan.mark.pittman@gmail.com>
 Jonathan Rudenberg <jonathan@titanous.com>
 Jonathan Wills <runningwild@gmail.com>