SubmittingPatches: add section for iterating patches

SubmittingPatches: add section for iterating patches

Add a section to explain how to work around other in-flight patches and
how to navigate conflicts which arise as a series is being iterated.
This provides the necessary steps that users can follow to reduce
friction with other ongoing topics and also provides guidelines on how
the users can also communicate this to the list efficiently.

Co-authored-by: Junio C Hamano <gitster@pobox.com>
Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>