update description

readme now reads:

_`mq()` is a [Sass](http://sass-lang.com/ "Sass - Syntactically Awesome Stylesheets") mixin that helps you compose media queries in an elegant way. It was developed in house at [the Guardian](http://www.theguardian.com/uk?view=mobile), and is now also in use at BBC Sport and the Financial Times…_

- keywords or `px`/`em` values are compiled to `em`-based queries
- fallbacks are provided for older browsers (see [Mobile-first Responsive Web Design and IE8] (http://www.theguardian.com/info/developer-blog/2013/oct/14/mobile-first-responsive-ie8) on the Guardian's developer blog).