manual: contributing: modify commit message template

This patch updates the commit message template provided in the manual.
The new template emphasizes the importance of providing a description of the
problem, the root cause, and the solution, and the fact that all this should
be wrapped at 72 characters.

Suggested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>