CONTRIBUTORS: add googler Michael Shields

R=golang-dev, dsymonds
CC=golang-dev, mshields
https://golang.org/cl/5042045

CONTRIBUTORS

@@ -168,6 +168,7 @@ Maxim Ushakov <ushakov@google.com>
 Micah Stetson <micah.stetson@gmail.com>
 Michael Elkins <michael.elkins@gmail.com>
 Michael Hoisie <hoisie@gmail.com>
+Michael Shields <mshields@google.com>
 Michael T. Jones <mtj@google.com> <michael.jones@gmail.com>
 Miek Gieben <miek@miek.nl> <remigius.gieben@gmail.com>
 Mikael Tillenius <mikti42@gmail.com>