docs: clarify that `box-sizing: border-box` is always required on resizable elements
docs: clarify that `box-sizing: border-box` is always required on resizable elements

Closes #61

mattlewis92/angular-resizable-element,mattlewis92/angular2-resizable,mattlewis92/angular-resizable-element,mattlewis92/angular2-resizable,mattlewis92/angular2-resizable,mattlewis92/angular-resizable-element