add a README for the dzslides backend

This a rough attempt to add some documentation for the dzslides backend (heavily inspired from the deckjs one).

My knowledge is quite limited on this subject, but I managed to make it work based on the deckjs example and what I had with Asciidoc, with no major regression on one of my slidedeck (which is now rendering 15x faster thx to asciidoctor!)

That should be a beginning for issue #5. What do you think?