Documentation: extend guidance for submitting patches

Documentation: extend guidance for submitting patches

Before submitting patches on the mailing list, it is often a good idea
to check for previous related discussions or if similar work is already
in progress. This enables better coordination amongst contributors and
could avoid duplicating work.

Additionally, it is often recommended to give reviewers some time to
reply to a patch series before sending new versions. This helps collect
broader feedback and reduces unnecessary churn from rapid rerolls.

Document this guidance in "Documentation/SubmittingPatches" accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>