d: add additional examples

Add additional examples of what a nftables firewall is.

The basic.nft file provides a default configuration similar to what one
would expect in a iptables firewall.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>
d/lintian-overrides: false positive

Lintian reports a false positive about the build flags.

Signed-off-by: Arturo Borrero Gonzalez <arturo.borrero.glez@gmail.com>