docs: set default registry to containers-registries.conf or add instructions to manually update it

[kimminss0]

The following instruction from the documentation on podman.io and the package messages from FreeBSD's port/pkg fails:

It is possible to run many Linux container images using FreeBSD's Linux emulation:

$ sudo sysrc linux_enable=YES
$ sudo service linux start
$ sudo podman run --rm --os=linux alpine cat /etc/os-release | head -1
NAME="Alpine Linux"

Here is how it fails:

# podman run --rm --os=linux alpine cat /etc/os-release
Error: short-name "alpine" did not resolve to an alias and no unqualified-search registries are defined in "/usr/local/etc/containers/registries.conf"

As related to #9, this issue seems to occur due to missing defaults for unqualified-search-registries in the configuration file, as described in man 5 containers-registries.conf. The instruction succeeded when I updated the configuration manually.

It would be nice to add some default entires such as ["quay.io", "docker.io"]. If that is not possible, at least, the documentation and package messages should include instructions on how to update the registries.conf file to add these repositories manually.