Markdown: References and inspiration
Note that tools like pandoc uses extended versions of markdown. Eg how to handle tables is not part of the original markdown.
The syntax of markdown is described at John Grubers homepage
There is also a markdown cheatsheet.
In practice markdown documents are often transformed using eg Pandoc and so The Pandoc User Guide is quite often necessary if something special has to be accomplished.
That could eg be how to integrate math formulas in the markdown document.
A homepage giving a quick overview of the markdown syntax is here.
Another great source of inspiration on how to use markdown and Pandoc is the article by Tenen and Wythoff