SubmittingPatches: discourage new trailers

SubmittingPatches: discourage new trailers

There seems to be consensus amongst the core Git community on a working
set of common trailers, and there are non-trivial costs to people
inventing new trailers (research to discover what they mean/how they
differ from existing trailers) such that inventing new ones is generally
unwarranted and not something to be recommended to new contributors.

Suggested-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>