Document that postgrest is sometimes unable to bind to localhost

[begriffs]

It appears in PostgREST/postgrest#899 that centos7 may not include a localhost alias in its /etc/hosts file, so postgrest is unable to bind there. The default binding of *4 does work, but investigate if there's a suitable alternative for people who want to host locally but not expose the server to the outside world. Perhaps 127.0.0.1 works. Document this in the configuration section.

[begriffs]

Also include special connection options listed in HostDataPreference

• * means HostAny - "any IPv4 or IPv6 hostname"
• *4 means HostIPv4 - "any IPv4 or IPv6 hostname, IPv4 preferred"
• !4 means HostIPv4Only - "any IPv4 hostname"
• *6 means HostIPv6@ - "any IPv4 or IPv6 hostname, IPv6 preferred"
• !6 means HostIPv6Only - "any IPv6 hostname"