SubmittingPatches: Clarify the Signed-off-by rules

The wording of the Signed-off-by rules could be read as stating that
S-O-B should only be added when the submitter considered the patch
ready for inclusion in git.git.

We also want Signed-off-by to be used for e.g. RFC patches, in case
someone wants to dig an old patch out of the archive and improve
it. Change the wording to recommend a Signed-off-by for all submitted
patches.

The problem with the wording came up in the "[PATCH/RFC] Hacky version
of a glob() driven config include" thread[1]. Bert Wesarg suggested[2]
that it be removed to avoid confusion, which this change implements.

  1. <1273180440-8641-1-git-send-email-avarab@gmail.com>
  2. <AANLkTimziTKL13VKIOcaS1TX1F_xvTVjH8Q398Yx36Us@mail.gmail.com>

Suggested-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -13,11 +13,9 @@ Checklist (and a short version for the impatient):
 		  not "changed" or "changes".
 		- includes motivation for the change, and contrasts
 		  its implementation with previous behaviour
-	- if you want your work included in git.git, add a
-	  "Signed-off-by: Your Name <you@example.com>" line to the
-	  commit message (or just use the option "-s" when
-	  committing) to confirm that you agree to the Developer's
-	  Certificate of Origin
+	- add a "Signed-off-by: Your Name <you@example.com>" line to the
+	  commit message (or just use the option "-s" when committing)
+	  to confirm that you agree to the Developer's Certificate of Origin
 	- make sure that you have tests for the bug you are fixing
 	- make sure that the test suite passes after your commit