fe80::/10 might not be safe on some platforms

[artizirk]

It might be worth to add a note to the README that on some platforms (*BSD) using fe80::/10 instead of /64 might be problematic.

From Wireguard mailinglist

FreeBSD and probably other *BSD/macOS use those unused 56 bits to
store the link scope_id. And support nonstandard fe80:1::30/64 notation
instead of fe80::30%1/64 to specify the scope.
https://stackoverflow.com/a/5891805/2303328
https://github.com/freebsd/freebsd/blob/76f9308e3e2b80e95630efcdd994f3c133806bf4/share/doc/IPv6/IMPLEMENTATION#L427
https://github.com/freebsd/freebsd/blob/e9a39e0c3c22543812afd4de74d1d0ad6782100b/sys/netinet6/scope6.c#L363