Also, add a script for future updates.

Change-Id: I2565d1f26532b9dd7cf9d8ce198ba08fb3d53407
Reviewed-on: https://go-review.googlesource.com/c/149604
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>