manual: contributing: don't recommend leaving out changelog from patches

Currently, the manual states that if you send a series and include a
changelog in the introduction mail (0 of x), it is not necessary to include
a changelog on the individual patches.
This is not always convenient: for large patch series it requires going back
to the introduction mail when opening each patch, to understand what
changed.
This patch rewords the paragraph accordingly.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>