Document a weird decision in RFC 8209

Document a weird decision in RFC 8209

The subject commonName of a BGPsec Router Certificate is RECOMMENDED to
be "CN=ROUTER-%08x", asn. It thus made perfect sense to deviate from
RFC 6487 and support encoding this as a UTF8String... We have three such
certs in the wild, so punt on complicating the logic at least until the
point where we need more than the fingers of one hand to count them.

ok claudio