SubmittingPatches: explain why 'next' and above are inappropriate base

SubmittingPatches: explain why 'next' and above are inappropriate base

The 'next' branch is primarily meant to be a testing ground to make
sure that topics that are reasonably well done work well together.
Building a new work on it would mean everything that was already in
'next' must have graduated to 'master' before the new work can also
be merged to 'master', and that is why we do not encourage basing
new work on 'next'.

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