Add FreeBSD information to the README

Author: 180watt

readme.md

@@ -58,6 +58,12 @@ If you're running `FreeBSD 11.3` or greater, you can install the package with:
 # pkg install vkquake
 ```
 
+Alternatvely, you can build vkQuake with FreeBSD's port collection:
+```console
+$ cd /usr/ports/games/vkquake
+# make install
+```
+
 ### Quake '2021 re-release'
 
 vkQuake has initial support for playing the 2021 re-release content. Follow installation instructions as above but copy the files into the rerelease folder.