Add HTML to the list of supported language in the docs (#5427)

docs/index.md

@@ -7,12 +7,15 @@ Prettier is an opinionated code formatter with support for:
 
 - JavaScript, including [ES2017](https://github.com/tc39/proposals/blob/master/finished-proposals.md)
 - [JSX](https://facebook.github.io/jsx/)
+- [Angular](https://angular.io/)
+- [Vue](https://vuejs.org/)
 - [Flow](https://flow.org/)
 - [TypeScript](https://www.typescriptlang.org/)
 - CSS, [Less](http://lesscss.org/), and [SCSS](http://sass-lang.com)
+- [HTML](https://en.wikipedia.org/wiki/HTML)
 - [JSON](http://json.org/)
 - [GraphQL](http://graphql.org/)
-- [Markdown](http://commonmark.org/), including [GFM](https://github.github.com/gfm/)
+- [Markdown](http://commonmark.org/), including [GFM](https://github.github.com/gfm/) and [MDX](https://mdxjs.com/)
 - [YAML](http://yaml.org/)
 
 It removes all original styling[\*](#footnotes) and ensures that all outputted code conforms to a consistent style. (See this [blog post](http://jlongster.com/A-Prettier-Formatter))