Add a tip to ease agnoster first setup

README.md

@@ -78,7 +78,9 @@ ZSH_THEME="robbyrussell"
 To use a different theme, simply change the value to match the name of your desired theme. For example:
 
 ```shell
-ZSH_THEME="agnoster" # (this is one of the fancy ones)
+ZSH_THEME="agnoster" # (this is one of the fancy ones ;
+                     # you might need to install a specific font on your console's host for this to work ;
+                     # see https://github.com/powerline/fonts/tree/master/DejaVuSansMono)
 ```
 
 Open up a new terminal window and your prompt should look something like...