README.markdown: add archive note
README.markdown: add archive note

Change-Id: I2109156753cecc7d4664a47ecba096fdf9bc86b6
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/282132
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>

golang/gddo,golang/gddo,golang/gddo,golang/gddo