docs: adding an example with visual feedback for the button

[nruth]

I guess most people will want the button to dance a bit when clicked. Since the run loop is or can be involved it might be good to have an idiomatic example in the readme. I'm not sure if this is idiomatic, but thought it might be a starting point.

    {{#zero-clipboard text=TEXT}}
      <button class="btn btn-primary" type="button">
        <i class="glyphicon glyphicon-duplicate"></i>
        <span class='copy-button-text'>
          Copy
        </span>
      </button>
    {{/zero-clipboard}}

import ZeroClipboard from 'ember-cli-zero-clipboard/components/zero-clipboard';
import Ember from 'ember';

export default ZeroClipboard.extend({
  actions: {
    afterCopy: function(){
      var startingText = Ember.$('.copy-button-text').first().text();
      Ember.$('.copy-button-text').text('Copied');
      Ember.run.later(window, function(){
        Ember.$('.copy-button-text').text(startingText);
      }, 2000);
    }
  }
});