docs: Document FreeBSD package in installation section

Author: taiki-e

README.md

@@ -547,6 +547,15 @@ You can install cargo-hack from the [extra repository](https://archlinux.org/pac
 pacman -S cargo-hack
 ```
 
+<!-- omit in toc -->
+### Via ports (FreeBSD)
+
+You can install cargo-hack from the [official ports](https://www.freshports.org/devel/cargo-hack):
+
+```sh
+pkg install cargo-hack
+```
+
 <!-- omit in toc -->
 ### Via other package managers