Add a big, glaring, WARNING that fork+exec on macOS is crashy

[dvarrazzo]

A problem that setproctitle might contribute, but not a setproctitle issue.

See #113 from this comment on.

The warning is not very prominent in Python docs, so let's make it more visible on our side.

[dlenski]

I think this is a great idea. Macos users of vpn-slice have also run into this issue, in dlenski/vpn-slice#159

The workaround suggested #113 (comment) should probably be mentioned as well, even though it can't always be applied:

It is possible to use setproctitle "safely" if the import setproctitle is done after the fork in child/parent.

(That said, I am completely on board with what you wrote in #113 @dvarrazzo: "I am not a macOS user, and personally I think nobody should." https://dlenski.github.io/macOS_nope.html 😄)