SubmittingPatches: note that trailer order matters

SubmittingPatches: note that trailer order matters

It matters where you put new trailers: they should be added in
chronological order, and each person who passes on a patch should add
their s-o-b last. You are signing off on the patch as well as the whole
message up to that point.

This also makes it clear who added what:

    Acked-by: The Reviewer <redacted>
    Signed-off-by: The Contributor <redacted>
    Acked-by: The (Late) Reviewer <redacted>
    Signed-off-by: The Maintainer <redacted>
The first ack was added by the contributor and the second one was added
by the maintainer.

Helped-by: Junio C Hamano <redacted>
Signed-off-by: Kristoffer Haugsbakk <redacted>
Signed-off-by: Junio C Hamano <redacted>