doc/README.md: explain new release note system

doc/README.md: explain new release note system

This is the first CL in a sequence that adds support
for generating release notes from fragments.

The actual generator will live elsewhere, in x/build.
This repo will hold the content and some validity
checks.

For #64169.

Change-Id: Iaa8d9ad96393ab9433170b3cfa47334837f3f691
Reviewed-on: https://go-review.googlesource.com/c/go/+/542515
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>