Add better installation instructions for old versions

Looking at how many issues are resolved by installing the correct version of Bourbon or Sass, a little more help in the description of the software seems appropriate.

This adds a paragraph to readme.md describing what (alternative implementations of Sass like libsass and sass-rails and some Frameworks (Compass, ZURB Foundation)) is affected and how to install the older version of Bourbon.

([Compass uses Sass 3.2.19](https://github.com/chriseppstein/compass/blob/stable/Gemfile#L11))
([The Foundation guides use libsass](http://foundation.zurb.com/docs/sass.html#using-foundation-with-grunt-libsass))