README.markdown: add archive note Change-Id: I2109156753cecc7d4664a47ecba096fdf9bc86b6 Reviewed-on: https://go-review.googlesource.com/c/gddo/+/282132 Trust: Julie Qiu <julie@golang.org> Run-TryBot: Julie Qiu <julie@golang.org> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>