README: make installation step 1 more accurate

Based on a patch by George Angelopoulos <george@usermod.net>.

Closes #149.

README.md

@@ -16,10 +16,12 @@ How to install
 
 ### In your ~/.zshrc
 
-* Download the script or clone this repository:
+* Clone this repository:
 
         git clone git://github.com/zsh-users/zsh-syntax-highlighting.git
 
+  (or [download a snapshot](https://github.com/zsh-users/zsh-syntax-highlighting/archive/master.tar.gz))
+
 * Source the script **at the end** of `~/.zshrc`:
 
         source /path/to/zsh-syntax-highlighting/zsh-syntax-highlighting.zsh