push: document the future default change for push.default (matching -> simple)

push: document the future default change for push.default (matching -> simple)

It is too early to start warning loudly about the future default change
in favor of 'simple', since many users use different versions of Git, and
would be harmed if we advised them to explicitely set
'push.default=simple' when using old versions of Git.

Still, we want to document the upcomming change so that:

* Users who may be affected by the change get one more chance to know it
  in advance.

* We actually commit to changing the default, and avoid repeating past
  errors.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>