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>