pkg-readme addition

[xhci]

In the pkg-readme, it should also be mentioned that net.inet.ip.mforwarding needs to be enabled.

net.inet.ip6.mforwarding is also mentioned in the multicast(4) man page, but doesn't seem to exist (?).

On second thought, there may be a of rule I was missing, but the above sysctl did fix the issue for me. I will try to investigate further and report back.