Add list of bugs in cmark-gfm to readme

All of these are solved here.

readme.md

@@ -20,6 +20,7 @@
     *   [`gfmFootnote()`](#gfmfootnote)
     *   [`gfmFootnoteHtml(options?)`](#gfmfootnotehtmloptions)
     *   [`HtmlOptions`](#htmloptions)
+*   [Bugs](#bugs)
 *   [Authoring](#authoring)
 *   [HTML](#html)
 *   [CSS](#css)
@@ -254,6 +255,36 @@ This label is used in the [`aria-label`][aria-label] attribute on each
 backreference (the `↩` links).
 It affects users of assistive technology.
 
+## Bugs
+
+GitHub’s own algorithm to parse footnote definitions contains several bugs.
+These are not present in this project.
+The issues relating to footnote definitions are:
+
+*   [Footnote reference call identifiers are trimmed, but definition
+    identifiers aren’t](https://github.com/github/cmark-gfm/issues/237)\
+    — initial and final whitespace in labels causes them not to match
+*   [Footnotes are matched case-insensitive, but links keep their casing,
+    breaking them](https://github.com/github/cmark-gfm/issues/239)\
+    — using uppercase (or any character that will be percent encoded) in
+    identifiers breaks links
+*   [Colons in footnotes generate links w/o
+    `href`](https://github.com/github/cmark-gfm/issues/250)\
+    — colons in identifiers generate broken links
+*   [Character escape of `]` does not work in footnote
+    identifiers](https://github.com/github/cmark-gfm/issues/240)\
+    — some character escapes don’t work
+*   [Footnotes in links are
+    broken](https://github.com/github/cmark-gfm/issues/249)\
+    — while `CommonMark` prevents links in links, GitHub does not prevent
+    footnotes (which turn into links) in links
+*   [Footnote-like brackets around image, break that
+    image](https://github.com/github/cmark-gfm/issues/275)\
+    — images can’t be used in what looks like a footnote call
+*   [GFM footnotes: line ending in footnote definition label causes text to
+    disappear](https://github.com/github/cmark-gfm/issues/282)\
+    — line endings in footnote definitions cause text to disappear
+
 ## Authoring
 
 When authoring markdown with footnotes it’s recommended to use words instead