Explain the reason for fqdn hostnames

The install guide asks the user to use the fqdn in the hostname, but
doesn't explain why. This has been asked multiple times, so adding an
explanation there.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>