Add a direction about Sketch.

I wrote some direction around Sketch.app.
gulp-iconfont and Sketch Tools made me happy to make webfonts ;-)

- [Sketch](http://bohemiancoding.com/sketch/)
- [Sketch Tools](http://bohemiancoding.com/sketch/tool/) via [gulp-sketch](https://github.com/cognitom/gulp-sketch)
- gulp-iconfont
- [gulp-consolidate](https://www.npmjs.org/package/gulp-consolidate) for CSS