docs: add MacPorts install info

`rq` can now be installed from MacPorts: https://ports.macports.org/port/rq/

Author: herbygillot

doc/installation.md

@@ -43,7 +43,7 @@ example `pacaur`:
 
 This takes a while to install because `rq` will be built from source.
 
-## Mac OS X
+## macOS
 
 There is a Homebrew tap available.  Add it like this:
 
@@ -60,6 +60,12 @@ severely out of date):
 
     brew install rq
 
+On macOS, can now also install `rq` using [MacPorts](https://www.macports.org):
+
+    sudo port install rq
+    
+More info [here](https://ports.macports.org/port/rq/).
+
 ## Nix
 
 `rq` is available in nixpkgs. You can install it via `nix-env`: