Add matrix.org badge to README.md

Matrix.org is an interesting platform for collaboration and community building. 

Thanks to various clients it supports it's maybe better than gitter to keep people on track and have a community feeling, discuss changes and more.

Not not split up into two parties not knowing of each other, the Gitter channel and the Matrix channel are bridged. This helps to keep everyone informed while add more medias.

Signed-off-by: Christoph Kern <sheogorath@shivering-isles.com>

Author: SISheogorath

README.md

@@ -4,6 +4,7 @@ HackMD Community Edition
 [![Standard - JavaScript Style Guide][standardjs-image]][standardjs-url]
 
 [![Join the chat at https://gitter.im/hackmdio/hackmd][gitter-image]][gitter-url]
+[![#HackMD on matrix.org][matrix.org-image]][matrix.org-url]
 [![build status][travis-image]][travis-url]
 [![version][github-version-badge]][github-release-page]
 [![Help Contribute to Open Source][codetriage-image]][codetriage-url]
@@ -306,6 +307,8 @@ See more at [http://operational-transformation.github.io/](http://operational-tr
 
 [gitter-image]: https://badges.gitter.im/Join%20Chat.svg
 [gitter-url]: https://gitter.im/hackmdio/hackmd?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
+[matrix.org-image]: https://img.shields.io/badge/Matrix.org-%23HackMD@matrix.org-green.svg
+[matrix.org-url]: https://riot.im/app/#/room/#hackmd:matrix.org
 [travis-image]: https://travis-ci.org/hackmdio/hackmd.svg?branch=master
 [travis-url]: https://travis-ci.org/hackmdio/hackmd
 [github-version-badge]: https://img.shields.io/github/release/hackmdio/hackmd.svg