docs: warn user of auto-started NGINX service

NGINX systemd service will be started automatically after NGINX
package is installed in setup.sh. Provide a note for end user to
be aware of this behavior and stop it.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: I0b1331c32212b467e4c06246aec1db386e814ce6