Documentation/process: add Co-Developed-by: tag for patches with multiple authors

Sometimes a single patch is the result of multiple authors.  As git only
can have one "author" of a patch, it is still good to properly give
credit to the other developers of a commit.  To address this, document
the "Co-Developed-by:" tag which can be used to show other authors of
the patch.

Note, these other authors must also provide a Signed-off-by: tag as it
is their work that is being submitted here.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>