Mention that the script is not VirtualBox-specific

[falconepl]

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The README mentions that pf is a script that exposes ports between VirtualBox and the host when it comes to docker-machine being used on macOS. However, pf works perfectly fine if someone uses any other hypervisor for Docker, such as xhyve, the native hypervisor for macOS - docker-machine-driver-xhyve. It should be the same case with HyperKit, another virtualization based on Hypervisor.framework introduced in macOS 10.10 - docker-machine-driver-hyperkit.

After analyzing the script it is obvious that pf is not VirtualBox dependent (as everything is done through docker-machine CLI), but I think it could be mentioned in the README.md as well. What do you think @johanhaleby?

And by the way - thanks for the script! Works like a charm.

[johanhaleby]

Oh yeah that's absolutely true, it's not tied to virtualbox or even docker. That was just what I used it for when creating the script :). If it's not too much to ask maybe you could edit the readme and submit a PR and add what you think would be an appropriate description?

[falconepl]

Sure, no problem! I will prepare a pull request for that