Add gif demo in README file
Add gif demo in README file

Add gif demo in README file.
Resolve issue #6 `demo gif for readme #6`.
Include the gif URL to show staple.js in action.
gif url: `https://cloud.githubusercontent.com/assets/1556583/19682970/75f8a4c0-9a86-11e6-8291-c085786ec6bb.gif`

minimill/pin.js,minimill/staple.js,minimill/pin.js,minimill/staple.js