Add sign off recommandition and definition of meaning.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/developer.texi

@@ -322,6 +322,10 @@ send a reminder by email. Your patch should eventually be dealt with.
     Does 'make fate' pass with the patch applied?
 @item
     Was the patch generated with git format-patch or send-email?
+@item
+    Did you sign off your patch? (git commit -s)
+    See url{http://kerneltrap.org/files/Jeremy/DCO.txt} for the meaning
+    of sign off.
 @item
     Is the patch against latest FFmpeg git master branch?
 @item