Add a link to AsciiDocBox in the website.

The [other][] live editor linked to from the [website][] is deactivated, which prompted me to create [AsciiDocBox][]. This pull request adds a link to that "try AsciiDoc in your browser" website.

This is [the thread][] introducing AsciiDocBox.

[other]: http://andrewk.webfactional.com/asciidoc.php
[website]: http://asciidoc.org/
[AsciiDocBox]: http://espadrine.github.io/AsciiDocBox/
[the thread]: https://groups.google.com/forum/#!topic/asciidoc/oezXYezBMgs