Add system runtime dependencies to install info.

Those are needed prior to `npm install`.

INSTALL.md

@@ -33,6 +33,12 @@ badge build passed :green .png > mybadge.png
 ```
 
 # Start the Server
+To run the server you will the following executables on your Path:
+- [PhantomJS](http://www.phantomjs.org/)
+- [Cairo](http://cairographics.org/) (runtime dependency for Canvas)
+
+On a OS X machine, [Homebrew](brew.sh) is a good package manager that will 
+allow you to install them.
 
 ```bash
 git clone git@github.com:badges/shields