docs(readme): ruby-sass download instructions

Document the need to download Ruby and install compass, whilst ruby-sass
is still used by default.

Closes #1243

readme.md

@@ -17,6 +17,13 @@ Install `yo`, `grunt-cli`, `bower`, `generator-angular` and `generator-karma`:
 npm install -g grunt-cli bower yo generator-karma generator-angular
 ```
 
+If you are planning on using Sass, you will need to first install Ruby and Compass:
+- Install Ruby by downloading from [here](http://rubyinstaller.org/downloads/) or use Homebrew
+- Install the compass gem:
+```
+gem install compass
+```
+
 Make a new directory, and `cd` into it:
 ```
 mkdir my-new-project && cd $_