add DHCPv6 client rule to example config

[jonny112]

I strongly recommend adding the following INPUT rule to the IPv6 example config:
proto udp saddr fe80::/10 daddr fe80::/10 sport dhcpv6-server dport dhcpv6-client ACCEPT;
As DHCPv6 is quite common and issues with it in absence of this rule can be tricky to debug, it would be very helpful to have this as a hint for new setups.

See also here
https://unix.stackexchange.com/questions/593357/why-do-we-need-to-have-an-ip6tables-rule-for-dhcp6-to-work-by-contrast-ipv4-d
and here
https://git.launchpad.net/ufw/tree/conf/before6.rules