Add test file for GitHub footnote syntax (closes #370).

test/github-footnote-syntax.md

@@ -0,0 +1,20 @@
+# GitHub Footnote Syntax
+
+> Example from <https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#footnotes>
+
+Here is a simple footnote[^1].
+
+A footnote can also have multiple lines[^2].
+
+You can also use words, to fit your writing style more closely[^note].
+
+[^1]: My reference.
+[^2]: Every new line should be prefixed with 2 spaces.
+  This allows you to have a footnote with multiple lines.
+[^note]:
+    Named footnotes will still render with numbers instead of the text but allow easier identification and linking.
+    This footnote also has been made with a different syntax using 4 spaces for new lines.
+
+<!-- markdownlint-configure-file {
+  "MD013": false
+} -->