add manual nameserver code example

Example 1: resolvconf set dns

sudo systemctl disable systemd-resolved.service
sudo gedit /etc/resolvconf/resolv.conf.d/head 
# add
nameserver 8.8.8.8
nameserver 8.8.4.4

sudo resolvconf --enable-updates
sudo resolvconf -u

Example 2: resolvconf set dns

$ sudo systemctl start resolvconf.service
$ sudo systemctl enable resolvconf.service
$ sudo systemctl status resolvconf.service