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 Blackfin-specific values

 CT_DoArchTupleValues() {

     # The architecture part of the tuple:

     CT_TARGET_ARCH="bfin"

     # gcc ./configure flags

     CT_ARCH_WITH_ARCH=

     CT_ARCH_WITH_ABI=

     CT_ARCH_WITH_CPU=

     CT_ARCH_WITH_TUNE=

     CT_ARCH_WITH_FPU=

     CT_ARCH_WITH_FLOAT=

 }