Not sure if this is the correct location for this issue as it concerns https://github.github.com/gfm/ rather than the implementation here, however I can't find a better place for it.

There are some omissions in the official specification for "GitHub Flavored Markdown" for syntax that is supported and mentioned elsewhere on GitHub.

In this case, footnote syntax is supported and referenced in Basic writing and formatting syntax#Footnotes, but is not part of the official specification.

At the very least this means:

• supporting GitHub Flavored Markdown becomes difficult for implementers (who have to dig into the source or search out other documentation to fully cover what is supported)
• using GitHub Flavored Markdown becomes difficult for users (who don't know what is actually supported and how the Markdown will be interpreted by any given tool that claims gfm support)

If this is intentional, then this should be stated somewhere in both the specification and the readme, if it is not, the specification needs updated.

See also #220 for more specification omissions.