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>

Documentation/process/5.Posting.rst

@@ -213,6 +213,11 @@ The tags in common use are:
    which can be found in Documentation/process/submitting-patches.rst.  Code without a
    proper signoff cannot be merged into the mainline.
 
+ - Co-Developed-by: states that the patch was also created by another developer
+   along with the original author.  This is useful at times when multiple
+   people work on a single patch.  Note, this person also needs to have a
+   Signed-off-by: line in the patch as well.
+
  - Acked-by: indicates an agreement by another developer (often a
    maintainer of the relevant code) that the patch is appropriate for
    inclusion into the kernel.