Added link to powerline (ohmyzsh#6352)

README.md

@@ -79,10 +79,11 @@ To use a different theme, simply change the value to match the name of your desi
 
 ```shell
 ZSH_THEME="agnoster" # (this is one of the fancy ones)
-# you might need to install a special Powerline font on your console's host for this to work
 # see https://github.com/robbyrussell/oh-my-zsh/wiki/Themes#agnoster
 ```
 
+_Note: many themes require installing the [Powerline Fonts](https://github.com/powerline/fonts) in order to render properly._
+
 Open up a new terminal window and your prompt should look something like this:
 
 ![Agnoster theme](https://cloud.githubusercontent.com/assets/2618447/6316862/70f58fb6-ba03-11e4-82c9-c083bf9a6574.png)