Add HTML to Markdown

README.md

 * [Parsedown](https://github.com/erusev/parsedown) - Another Markdown parser.
 * [Ciconia](https://github.com/kzykhys/Ciconia) - Another Markdown parser that supports Github flavoured Markdown.
 * [Cebe Markdown](https://github.com/cebe/markdown) - An fast and extensible Markdown parser.
+* [HTML to Markdown](https://github.com/thephpleague/html-to-markdown) - Converts HTML into Markdown.
 * [HTML5 PHP](https://github.com/Masterminds/html5-php) - An HTML5 parser and serializer library.
 * [Emoji](https://github.com/heyupdate/Emoji) - A library that converts unicode characters and names into emoji images.