add note to use bootstrap-sass, for compatible with glyphicons.

Hi.

I found a problem when using "bootstrap-sass" gem with middleman (3.3.2); glyphicons are not working.

In this document, we should add only this line to Gemfile

gem "bootstrap-sass", :require => false
but, I think this line are also needed to use bootstrap-sass with middleman... isn't it?

# in config.rb
require "bootstrap-sass/sass_functions.rb"
I think it's a trap in the document.