Add note to the readme about bash versions pre 4.0

Readme.md

@@ -47,6 +47,9 @@ about your system and prints it to the terminal next to an image of your choice!
 These are the script's required dependencies
 
 -  Bash **4.0+**
+    - For those of you on OS X not wanting to update bash, the script
+      also works with zsh. All you have to do is change the shebang at
+      the top to ```#!/bin/zsh```.
 -  Text formatting, dynamic image size and padding: tput
 -  Uptime detection: procps or procps-ng