Footnote syntax not documented in Markdown help documentation

Suggested label: ~Documentation

In at least GitLab version 8.12.4(on GitLab.com), the footnote syntax for GitLab Flavored Markdown is not documented on the /help/user/markdown.md page.

The footnote syntax is as follows:

Test[^1]

[^1]: This is a footnote.

Which produces:

Test¹

1. This is a footnote. ↩