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>

# Compute M68k-specific values

CT_DoArchTupleValues() {

    :

}