Fixed instructions for using Foundation with an existing project

Hi,

I'm not a Ruby programmer in any way, shape, or form; but in order to use Foundation with an existing project I had to perform these extra steps, otherwise compass would croak with

```
LoadError on line ["161"] of /Users/pfig/.rvm/gems/ruby-2.0.0-p247/gems/compass-0.12.2/lib/compass/configuration/data.rb: cannot load such file -- zurb-foundation
```