README: instructions for installing via MacPorts (#539, @herbygillot)

README.md

@@ -58,6 +58,7 @@ You have several options there:
 
 - use [Homebrew](https://brew.sh): `brew install k3d` (Homebrew is available for MacOS and Linux)
   - Formula can be found in [homebrew/homebrew-core](https://github.com/Homebrew/homebrew-core/blob/master/Formula/k3d.rb) and is mirrored to [homebrew/linuxbrew-core](https://github.com/Homebrew/linuxbrew-core/blob/master/Formula/k3d.rb)
+- install via [MacPorts](https://www.macports.org): `sudo port selfupdate && sudo port install k3d` (MacPorts is available for MacOS)
 - install via [AUR](https://aur.archlinux.org/) package [rancher-k3d-bin](https://aur.archlinux.org/packages/rancher-k3d-bin/): `yay -S rancher-k3d-bin`
 - grab a release from the [release tab](https://github.com/rancher/k3d/releases) and install it yourself.
 - install via go: `go install github.com/rancher/k3d` (**Note**: this will give you unreleased/bleeding-edge changes)