doc/README.md: mention adding the proposal issue

doc/README.md: mention adding the proposal issue

Inform users that every accepted proposal should be mentioned
in the release notes to avoid generating a TODO.

Also, clarify instructions for the release team.

For #64169.

Change-Id: I03921739af31fc93f4b5808b2c7c98e28ad3da11
Reviewed-on: https://go-review.googlesource.com/c/go/+/584615
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>