Adds option to disable auto update prompt

I noticed this option was missing from the template. When I followed the directions in README to enable it, I put it below line 59 without noticing it wouldn't have any effect by then. Prevent others from making a rookie mistake like mine.

templates/zshrc.zsh-template

@@ -19,6 +19,9 @@ ZSH_THEME="robbyrussell"
 # Uncomment the following line to disable bi-weekly auto-update checks.
 # DISABLE_AUTO_UPDATE="true"
 
+# Uncomment the following line to automatically update without prompting.
+# DISABLE_UPDATE_PROMPT="true"
+
 # Uncomment the following line to change how often to auto-update (in days).
 # export UPDATE_ZSH_DAYS=13