People may be surprised to find out nothing happens if they just
open a presentation html with an external markdown file using
the file:/// protocol, let them know they need a server for that.
External markdown: Mention that a http server is required
PS: It would be perhaps good to also mention that --- surrounded by blank lines is the default vertical separator but I suppose you want to keep the document as short as possible and must people can figure it out anyway or just set their own separator ....
There's a few features that require the presentation to be running through a web server so it needs to be messaged more broadly rather than specifically for external markdown. Will make sure this is covered as the recommended setup for the whole framework as part of #357