C+A: add Matthew Cottingham (Individual CLA)

Done by addca, but codereview failed with a Python stacktrace,
so submitting by hand.

R=golang-dev
CC=golang-dev
https://golang.org/cl/16650043

AUTHORS

@@ -218,6 +218,7 @@ Mats Lidell <mats.lidell@cag.se>
 Matt Jibson <matt.jibson@gmail.com>
 Matt Joiner <anacrolix@gmail.com>
 Matt Reiferson <mreiferson@gmail.com>
+Matthew Cottingham <mattcottingham@gmail.com>
 Matthew Horsnell <matthew.horsnell@gmail.com>
 Micah Stetson <micah.stetson@gmail.com>
 Michael Chaten <mchaten@gmail.com>

CONTRIBUTORS

@@ -322,6 +322,7 @@ Matt Jibson <matt.jibson@gmail.com>
 Matt Joiner <anacrolix@gmail.com>
 Matt Jones <mrjones@google.com>
 Matt Reiferson <mreiferson@gmail.com>
+Matthew Cottingham <mattcottingham@gmail.com>
 Matthew Dempsky <mdempsky@google.com>
 Matthew Horsnell <matthew.horsnell@gmail.com>
 Maxim Pimenov <mpimenov@google.com>