Issue title: Documentation about most used markdown extension
 Issue body: I propose to add a  small section to the documentation about markup extensions.

1. An example use of MARKDOWNX_MARKDOWN_EXTENSIONS = ['markdown.extensions.nl2br', ]
which in imho is the most wanted extension.
2. a link to a page where people can [learn more about markdown extensions](https://pythonhosted.org/Markdown/extensions/index.html)
 Comments: 
 Comment 0: Can do :)
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