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>
1 file changed