doc: add maintainer role about replying questions

The first line of the MAINTAINERS file are:
"
The intention of this file is to provide a set of names that we can rely on
for helping in patch reviews and questions.
"

Unfortunately, some maintainers do not endorse their role for questions
asked on the mailing list.
Hope making it clear in the contribution guide will enforce more
responsive maintainer participation.

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: John McNamara <john.mcnamara@intel.com>