manual: contributing: move section on patch reviews up and change intro

This patch moves the section about patch reviews and the
Tested-by/Reviewed-by/Acked-by tags upwards. Additionally, the first
paragraph is removed and replaced by another one. There are no other changes
in the text.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>