Mention markdown all in one extension

Author: MarvNC

CONTRIBUTING.md

@@ -78,3 +78,5 @@ run by `npm test` and the continuous integration process.
 - [.eslintrc.json](.eslintrc.json) rules are used for JavaScript files.
 - [.stylelintrc.json](.stylelintrc.json) rules are used for CSS files.
 - [.htmlvalidate.json](.htmlvalidate.json) rules are used for HTML files.
+
+In addition, the [Markdown All in One VSCode extension](https://github.com/yzhang-gh/vscode-markdown) is used for formatting markdown files and automatically updating the table of contents.