Add archive note about voila-reveal being part of core Voila

[jtpio]

Problem

The voila-reveal template was added to the core Voila repo in voila-dashboards/voila#932

Suggested Improvement

We should document this here in this repo, to inform users of voila==0.3 they don't need to install voila-reveal separately.

[SylvainCorlay]

I added a note in the readme about this, and moved the issues to the Voilà GitHub repository.

[jtpio]

Nice, thanks!