Call out that the GitHub Flavored Markdown spec is incomplete

Author: mkantor

content/packages-and-modules/contributing-packages-to-the-registry/about-package-readme-files.mdx

@@ -2,7 +2,7 @@
 title: About package README files
 ---
 
-We highly recommend including a `README.md` file in your package directory as it helps developers find your package on npm and have a good experience using your code in their projects. In most cases `README.md` files include directions for _installing_, _configuring_, and _using_ the code in your package; as well as any other information a user may find helpful. Just like in any GitHub repository, the `README.md` file will be rendered on the package's page. On [npmjs.com](https://www.npmjs.com/), the `README.md` is rendered as [GitHub Flavored Markdown](https://github.github.com/gfm/) via [GitHub's API](https://docs.github.com/en/rest/markdown/markdown).
+We highly recommend including a `README.md` file in your package directory as it helps developers find your package on npm and have a good experience using your code in their projects. In most cases `README.md` files include directions for _installing_, _configuring_, and _using_ the code in your package; as well as any other information a user may find helpful. Just like in any GitHub repository, the `README.md` file will be rendered on the package's page. On [npmjs.com](https://www.npmjs.com/), the `README.md` is rendered as [GitHub Flavored Markdown](https://github.github.com/gfm/)[^1] via [GitHub's API](https://docs.github.com/en/rest/markdown/markdown).
 
 An npm package `README.md` file **must** be in the root-level directory of the package.
 
@@ -33,4 +33,6 @@ The `README.md` file will only be updated on the package page when you publish a
    npm publish
    ```
 
+[^1]: Newer GFM features like footnotes are supported, even though they aren't in the spec (as of 2025).
+
 [markdown-link]: https://guides.github.com/features/mastering-markdown/#what