SubmittingPatches: clarify DCO is our --signoff rule

SubmittingPatches: clarify DCO is our --signoff rule

The description on sign-off and DCO was written back in the days
where there was only a choice between "use sign-off and it means the
contributor agrees to the Linux-kernel style DCO" and "not using
sign-off at all will make your patch unusable".  These days, we are
trying to clarify that the exact meaning of a sign-off varies
project to project.

Let's be more explicit when presenting what _our_ rules are.  It is
of secondary importance that it originally came from the kernel
project, so move the description as a historical note at the end,
while cautioning that what a sign-off means to us may be different from
what it means to other projects contributors may have been used to.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Bradley M. Kuhn <bkuhn@sfconservancy.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>