Also, document process for updating vendored x/tools.

Change-Id: I826744603ae0752e508a6db7334a2bf9adaf1289
Reviewed-on: https://go-review.googlesource.com/c/149963
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>