Add entry to README about node-sass configuration

[peterblazejewicz]

Like:

To build that boilerplate with `node-sass` installed globally do following:

- install `node-sass` if not yet installed:
``bash
npm install -g node-sass
``
- run build command from command line:
``bash
node-sass stylesheets/main.scss dist/main.css 
``

Thanks!