> org-mode does indeed support footnotes[fn:1], but I was recently saddened to find that Github doesn't render them (it does render more basic org-mode features, though).

GitHub uses this org-ruby project[1] to render Org files. If someone loves Org and knows Ruby too (I don't), please open PR's on that repo to improve Org parsing.

[1]: https://github.com/wallyqs/org-ruby