document Acked-by:

Explain what we use Acked-by: for, and how it differs from Signed-off-by:
Acked-by: Dave Jones <davej@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/SubmittingPatches

@@ -340,8 +340,32 @@ now, but you can do this to mark internal company procedures or just
 point out some special detail about the sign-off. 
 
 
+13) When to use Acked-by:
 
-13) The canonical patch format
+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.
+
+If a person was not directly involved in the preparation or handling of a
+patch but wishes to signify and record their approval of it then they can
+arrange to have an Acked-by: line added to the patch's changelog.
+
+Acked-by: is often used by the maintainer of the affected code when that
+maintainer neither contributed to nor forwarded the patch.
+
+Acked-by: is not as formal as Signed-off-by:.  It is a record that the acker
+has at least reviewed the patch and has indicated acceptance.  Hence patch
+mergers will sometimes manually convert an acker's "yep, looks good to me"
+into an Acked-by:.
+
+Acked-by: does not necessarily indicate acknowledgement of the entire patch.
+For example, if a patch affects multiple subsystems and has an Acked-by: from
+one subsystem maintainer then this usually indicates acknowledgement of just
+the part which affects that maintainer's code.  Judgement should be used here.
+ When in doubt people should refer to the original discussion in the mailing
+list archives.
+
+
+14) The canonical patch format
 
 The canonical patch subject line is: