CONTRIBUTING: add guidelines regarding email
CONTRIBUTING: add guidelines regarding email

This adds guidelines around reaching the maintainers. Emails shouldn't
be sent to maintainers. GitHub mentions should be used instead.

Signed-off-by: Cristian Staretu <78db8d24f2e00709fded2f6f25e9a572f4b7462f@gmail.com>

aditirajagopal/docker,aditirajagopal/docker,aditirajagopal/docker