README: add MacPorts install info (#716)

Add info on installing `fastfetch` on macOS via MacPorts: https://ports.macports.org/port/fastfetch/

Author: herbygillot

README.md

@@ -42,8 +42,14 @@ Replace sudo with doas depending on what you use.
 
 ### macOS
 
+...via [HomeBrew](https://brew.sh):
+
 `brew install fastfetch`
 
+...via [MacPorts](https://www.macports.org):
+
+`sudo port install fastfetch`
+
 ### Windows
 
 `scoop install fastfetch`