Link screenshot to intro video

When I first saw your readme I tried clicking the screenshot to watch the video, and was surprised it took me to a page with the screenshot in isolation. This pull request makes the picture link to your youtube video.

Note: I had to use regular HTML instead of markdown to make it work. I could see how you might not like this, but I thought it was worth it for the user experience.

by the way, I :heart: this project -- very ingenious.