Document minimum system requirements

[afbjorklund]

Think you need to list minimum versions required, of the various system tools used:

• Python 3.5+ (?)

• OpenSSH 6.7+

Had some issues, when I tried to use older versions:

    **options,
             ^
SyntaxError: invalid syntax

https://docs.python.org/3/whatsnew/3.5.html#whatsnew-pep-448

PEP 448 extends the allowed uses of the * iterable unpacking operator and ** dictionary unpacking operator. It is now possible to use an arbitrary number of unpackings in function calls:

Bad local forwarding specification '/tmp/pypodman/podman.socket:/run/podman/io.podman'

https://www.openssh.com/txt/release-6.7

• ssh(1), sshd(8): Add support for Unix domain socket forwarding.
  A remote TCP port may be forwarded to a local Unix domain socket
  and vice versa or both ends may be a Unix domain socket.

Shouldn't be unreasonable to demand (?), but would be good to document anyway...