reame: add MacPorts instructions

Also, use "macOS" uniformly in README.md.

PR #152

Author: lperry

README.md

@@ -299,15 +299,22 @@ right and full outer join support too.
 
 ### Installation
 
-Binaries for Windows, Linux and Mac are available [from Github](https://github.com/BurntSushi/xsv/releases/latest).
+Binaries for Windows, Linux and macOS are available [from Github](https://github.com/BurntSushi/xsv/releases/latest).
 
-If you're a **Mac OS X Homebrew** user, then you can install xsv
+If you're a **macOS Homebrew** user, then you can install xsv
 from homebrew-core:
 
 ```
 $ brew install xsv
 ```
 
+If you're a **macOS MacPorts** user, then you can install xsv
+from the [official ports](https://www.macports.org/ports.php?by=name&substr=xsv):
+
+```
+$ sudo port install xsv
+```
+
 If you're a **Nix/NixOS** user, you can install xsv from nixpkgs:
 
 ```