docs: add Co-Developed-by docs

When Co-Developed-by tag was added, docs were only added to
Documention/process/5.Posting.rst and were not added to
Documention/process/submitting-patches.rst

Add documentation to Documention/process/submitting-patches.rst

Signed-off-by: Tobin C. Harding <me@tobin.cc>
[jc: tweaked commas in the subheading]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>