Initialize the doc/next directory for the next release by copying the
contents of doc/initial into it.
Also, rewrite doc/README.md to add release instructions and to separate
information for developers from information for the release team.
Change-Id: Ie1e5c2ddae7a8d301e9b2f8a68925519648aa84d
Reviewed-on: https://go-review.googlesource.com/c/go/+/557535
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
