SubmittingPatches: move discussion of Signed-off-by above "send"

SubmittingPatches: move discussion of Signed-off-by above "send"

Move the section discussing the addition of a SOB trailer above the
section that discusses generating the patch itself. This makes sense
as we don't want someone to go through the process of "git
format-patch", only to realize late that they should have used "git
commit -s" or equivalent.

This is a move-only change, no lines here are being altered, only
moved around.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>