doc: updated language about the AUTHORS/CONTRIBUTORS update
Reflect the process changes where AUTHORS and CONTRIBUTORS
files are updated automatically based on commit logs
and Google committers no longer need to do it manually
on the first contributors.
The documentation update will help to avoid requests to be
added from new contributors.
Change-Id: I67daae5bd21246cf79fe3724838889b929bc5e66
Reviewed-on: https://go-review.googlesource.com/10824 Reviewed-by: Rob Pike <r@golang.org>