The text says "if you can certify DCO then you add a Signed-off-by
trailer".  But it does not say anything about people who cannot or
do not want to certify.  A natural reading may be that if you do not
certify, you must not add the trailer, but it shouldn't hurt to be
overly explicit.

Signed-off-by: Junio C Hamano <gitster@pobox.com>