SubmittingPatches: discuss reviewers first

SubmittingPatches: discuss reviewers first

No matter how well someone configures their email tooling, understanding
who to send the patches to is something that must always be considered.
So discuss it first instead of at the end.

In the following commit we will clean up the (now redundant) discussion
about sending security patches to the Git Security mailing list.

Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>