Document supported speaker notes syntax

[cderv]

Following #45 (comment) , according to https://pandoc.org/MANUAL.html#speaker-notes

---
title: "Test"
output: 
  revealjs::revealjs_presentation:
    self_contained: false
    reveal_plugins: notes
---

# Main header

::: notes
Main header notes
:::

## Subheader 1

::: notes
Subheader 1 notes
:::

## Subheader 2

::: notes
Subheader  notes
:::