doc: document the description line for the commit message

Documented how to format description lines for packages patches.
Added a note on enforcing Signed-off-by lines.

Signed-off-by: Michael Hope <michael.hope@linaro.org>
[yann.morin.1998@anciens.enib.fr: further describe the desc line]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Gustavo Zacarias <gustavo@zacarias.com.ar>"

 reporter_url="http://sourceware.org/ml/crossgcc/2011-03/msg00028.html"

 reporter_comment=""