From the Github README:
Simply include the code on your pages and footnotes will be detected automatically and improved in the following ways:
Links to footnotes will be replaced with clickable/tappable buttons, making them substantially easier to hit.
Footnote content will appear in a popover directly beside the footnote button when it is clicked/tapped, which cuts out the annoying bouncing around the page that footnotes typically result in.
The active popovers will be resized and repositioned to ensure that they continue to be completely visible on-screen and aesthetically pleasing: this makes it perfect for mobile devices and responsive designs.
Supports the markup generated by MultiMarkdown by default.