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>

 python

 import sys

 sys.path.insert(0, '@@PREFIX@@/share/gcc-@@VERSION@@/python')

 from libstdcxx.v6.printers import register_libstdcxx_printers

 register_libstdcxx_printers (None)

 end