Update README.md - add libxkbfile as a dependency on FreeBSD (#61)

Author: yonas

README.md

@@ -6,7 +6,7 @@ Returns what characters are produced by pressing keys with different modifiers o
 * On Debian-based Linux: `sudo apt-get install libx11-dev libxkbfile-dev`
 * On Red Hat-based Linux: `sudo yum install libx11-devel.x86_64 libxkbfile-devel.x86_64 # or .i686`
 * On SUSE-based Linux: `sudo zypper install libX11-devel libxkbfile-devel`
-* On FreeBSD: `sudo pkg install libX11`
+* On FreeBSD: `sudo pkg install libX11 libxkbfile`
 
 ```sh
 npm install native-keymap