docs: add reference to release cycle discussion

docs: add reference to release cycle discussion

As it is coming up basically every release cycle of Xen, add a
reference to the discussion why the current release scheme has been
selected in the release management documentation.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>
Acked-by: Jan Beulich <jbeulich@suse.com>