Documentation: show how to switch to 16:9

[bast]

myslides.body.html:

+<textarea id="source">
+{{ markdown }}
+</textarea>
+
+<script src="{{ engine_root }}/remark.min.js" type="text/javascript"></script>
+
+<script type="text/javascript">
+  var slideshow = remark.create({ratio: '16:9'});
+</script>