Add breakpoint sizes to documentaion

The grid can be manipulated based on desktop, phone, tablet sizes. But the [documentation](http://www.getmdl.io/components/index.html#layout-section/grid) doesn't mention what size it considers these devices to be.

The [layout documentation](http://www.getmdl.io/components/index.html#layout-section) also contains the `mdl-layout--large-screen-only` and `mdl-layout--small-screen-only` classes, and it'd be great to list in the docs what size these get triggered.