AS OF MAY 2016, THIS PLUGIN IS NOT BEING MAINTAINED AND WILL NOT RECEIVE ANY FUTURE UPDATES. 
CONTACT ME IF YOU’D LIKE TO LIST AN ALTERNATIVE HERE.
Alternatives: 
– https://wordpress.org/plugins/footnotation/
This plugin provides an extremely easy way to add elegant looking footnotes to 
your posts. The syntax is natural, simple to understand, and degrades 
gracefully if the plugin is removed. Footnotes are linked unobtrusively and the 
note itself links back to the original text where the footnote is referenced.
Adding footnotes to a post is simple. Just type them inline in your post in 
square brackets like this:
<pre><code> [1. This is a footnote.]
</code></pre>
Each footnote must have a number followed by a period and a space and then the 
actual footnote. They don’t have to be unique but it is recommended. It doesn’t 
matter what the numbers are since the footnotes will be automatically 
renumbered when the post is displayed.
Footnotes can contain anything you’d like including links, images, etc. 
Footnotes are automatically linked back to the spot in the text where the note 
was made.
<h4>Settings</h4>
Only show footnotes on single post/page: This option will hide 
footnotes on the main blog page. Footnote numbers will still appear 
but link to the individual post/page URL.
Collapse footnotes until clicked: When checked, footnotes are 
hidden until manually expanded or a footnote number is clicked.
Note: Do not include square brackets [] inside the footnotes themselves.
Note: Footnote numbers don’t need to be unique but it is recommended, 
especially if the text is identical for multiple footnotes. If you have 
multiple footnotes with the exact same text and number then you’ll get weird 
and incorrect footnotes on your post.

FD Footnotes Plugin

Add elegant looking footnotes to your posts simply and naturally.

acusti

The <a href="https://wordpress.org/plugins/simple-footnotes/" rel="ugc">Simple Footnotes</a> plugin allows you to add simple, elegant footnotes on your site.
This tiny plugin makes it incredibly simple to create those footnotes, leaving nothing to teach the site’s editors (no need to explain shortcodes or special syntaxes). It adds a button to the TinyMCE editor toolbar in the second row (i.e. the “Kitchen Sink”) that users can click to open a textarea overlay where they can enter the footnote content and then insert it into the editor with the proper syntax.
<h3>Translation</h3>
<ul>
<li>French</li>
</ul>

Simple Footnotes Editor Button

Add a button to the TinyMCE editor toolbar to make it easy for users to add footnotes with the proper syntax for the Simple Footnotes plugin.

FD Footnotes Plugin vs Simple Footnotes Editor Button