Add an animated screenshot to the README

[toolness]

This is related to #9, except it's a shorter-term interim solution that involves just adding an animated GIF of the glossary on an existing site, rather than setting up a separate example site and recording it.

Here's an animated GIF I just took of the panel in action on the EITI site that can be used:

Wow, that is a lot bigger than I thought it'd be.

[stphnwlkr]

Nice. The size makes it very legible. Love the active link to the glossary item.