Make it clear that the demo's urls aren't bookmarkable

[mairatma]

This project's demo page is an example of an SPA web site that changes the browser's url to different values. If you refresh the browser on any url but the initial one you'll get a 404 though, which feels like a bug, though it isn't. This happens because the demo only implements the client side, so the server doesn't know about the other urls.

We should make that clear somewhere (maybe in the demo page itself).