SubmittingPatches: emphasize need to communicate non-default starting points
The phrase
and unless it targets the `master` branch (which is the default),
mark your patches as such.
is tightly packed with several things happening in just two lines of
text. It also feels like it is not that important because of the terse
treatment. This is a problem because (1) it has the potential to confuse
new contributors, and (2) it may be glossed over for those skimming the
docs.
Emphasize and elaborate on this guidance by promoting it to its own
separate paragraph.
Signed-off-by: Linus Arver <linusa@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>