Documentation/submitting-patches: Document RESEND tag on patches

Explain when a submitter should tag a patch or a patch series with the
"RESEND" tag.

This has been partially carved out from a tip subsystem handbook
patchset by Thomas Gleixner:

  https://lkml.kernel.org/r/20181107171010.421878737@linutronix.de



and incorporates follow-on comments.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>