UPDATING: ino64 upgrade should include COMPAT_FREEBSD11

The upgrade process requires COMPAT_FREEBSD11 to support the combination
of "old" userland and "new" kernel that exists after "make kernel" and
reboot. Mention this explicitly for those using custom kernel configs.
Once the "new" world is installed the COMPAT_FREEBSD11 could be removed
again, but that does not seem necessary to mention in UPDATING.

Reported by: kib
Sponsored by: The FreeBSD Foundation