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>

Documentation/process/submitting-patches.rst

@@ -510,8 +510,8 @@ tracking your trees, and to people trying to troubleshoot bugs in your
 tree.
 
 
-12) When to use Acked-by: and Cc:
----------------------------------
+12) When to use Acked-by:, Cc:, and Co-Developed-by:
+-------------------------------------------------------
 
 The Signed-off-by: tag indicates that the signer was involved in the
 development of the patch, or that he/she was in the patch's delivery path.
@@ -543,6 +543,11 @@ person it names - but it should indicate that this person was copied on the
 patch.  This tag documents that potentially interested parties
 have been included in the discussion.
 
+A 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.
+
 
 13) Using Reported-by:, Tested-by:, Reviewed-by:, Suggested-by: and Fixes:
 --------------------------------------------------------------------------