Summary: The discourse footnote plugin allows you to create markdown footnotes in your posts.

GitHub: https://github.com/discourse/discourse-footnote
Install: Follow the plugin installation guide.
Features
Footnotes have been discussed as an optional CommonMark extension for a while:
The general consensus is that the extension should allow for syntax such as:
some deep thought who needs a reference[^1]
[^1]: Here's the reference.
Discourse footnote re-packages the footnote markdown.it extension by @Vitaly as a Discourse plugin.
It supports both, single line and multi paragraph footnotes.
So, for example, this works.
We are our choices. [^sartre]
A lot of interesting words
He really said it. [^sartre]
[^sartre]: We are our choices.
Sartre, some time
some day
in the past
You can also use “inline” footnotes — not the inline expansion, but the note itself. Like this:
More wise sayings ^[Lincoln, or Einstein, or something]
and then you get:
More wise sayings
In a multi site environment each site can enable or disable the extension with the enable_markdown_footnotes setting.
Additionally you can either enable or disable “inline” expansion with display_footnotes_inline
Screenshots
This is how stuff looks without inline expansion:

This is how stuff looks with inline expansion:

Inline expansion is “quote” friendly and mobile friendly.
CHANGELOG
TODO