SubmittingPatches: discourage new trailers

There seems to be consensus amongst the core Git community on a working
set of common trailers, and there are non-trivial costs to people
inventing new trailers (research to discover what they mean/how they
differ from existing trailers) such that inventing new ones is generally
unwarranted and not something to be recommended to new contributors.

Suggested-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -356,8 +356,9 @@ If you like, you can put extra tags at the end:
 . `Tested-by:` is used to indicate that the person applied the patch
   and found it to have the desired effect.
 
-You can also create your own tag or use one that's in common usage
-such as "Thanks-to:", "Based-on-patch-by:", or "Mentored-by:".
+While you can also create your own trailer if the situation warrants it, we
+encourage you to instead use one of the common trailers in this project
+highlighted above.
 
 [[git-tools]]
 === Generate your patch using Git tools out of your commits.