some details for beginners using reveal.js and specific syntax used in hackmd's implementation

Slide-Options.md

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+## Slide Separators
+If you're getting started with reveal.js slides, there are a few things you need to know.
+
+There are two types of slides, those that transition horizontally and those that transition vertically (subslides).
+
+The following separators are used for each in the hackmd syntax:
+```
+# First Slide
+
+---
+
+# Next slide
+
+----
+
+## Subslide
+```
+as you can see, horizontal transitions are separated by `---` and vertical transitions by `----`
+
+## Basic YAML header
+It's possible to customise the slide options using the YAML header in the slide markdown.
+
+eg:
+```
+---
+title: Example Slide
+tags: presentation
+slideOptions:
+  theme: solarized
+  transition: 'fade'
+  # parallaxBackgroundImage: 'https://s3.amazonaws.com/hakim-static/reveal-js/reveal-parallax-1.jpg'
+---
+```
+make sure to have two spaces only at the start of the listed slide options.
+
+you can comment out options with a `#`
+
+