Ryan

Adds a section to the bottom of your pages and posts listing any external 
links.
Only links starting with http or https are included so ftp, mailto, etc 
will not be included in the list; although it would be pretty trival to 
edit the plugin to allow them to. Likewise any links that start with your 
blog URL are also excluded.
For more information, you can visit <a href="http://www.nutt.net/tag/link-footnotes/" rel="nofollow ugc">Nutt.net</a>

Link Footnotes

Adds a footnotes section to your posts with any external links

maurobringolf

This plugin gives you a shortcode to make numbered references to footnotes, directly from your post content. Footnotes are listed and enumerated automatically at the end of your post 
content, reusing all existing styles of your site. All references become anchor links guiding the user to the footnote section of the post.
We would love to hear and discuss your feedback in a <a href="https://wordpress.org/support/plugin/mnml-footnotes/reviews/" rel="ugc">review</a>. If you want to report a bug or have a feature request, please do so via the <a href="https://github.com/maurobringolf/mnml-footnotes" rel="nofollow ugc">GitHub repository</a>.
<h3>Usage</h3>
To make a new footnote reference in your content you can use the shortcode mnml_footnote in the following way:
This is your regular post content and the shortcode by this plugin looks like this [mnml_footnote] Easy, right? [/mnml_footnote]
Everything inside the shortcode will be moved into a footnote and replaced by a numbered reference link to it. In this example the footnote content would be “Easy, right?”.

Link Footnotes vs MNML Footnotes