Add hackmd to editors section

README.md

@@ -830,6 +830,7 @@ See https://github.com/n1trux/awesome-sysadmin#monitoring and https://github.com
   * [savepad](https://github.com/novavex/textpad) - A minimalist notepad based on notepad.cc. `MIT` `PHP`
   * [ShareLaTex](https://www.sharelatex.com/) - A web-based collaborative LaTeX editor. ([Source Code](https://github.com/sharelatex/sharelatex)) `AGPLv3` `Ruby`
   * [Turtl](https://turtl.it/) - A totally private personal database and note taking app. ([Source Code](https://github.com/turtl)) `GPLv3` `CommonLisp`
+  * [HackMD](https://hackmd.io) - A feature-rich collaborative online markdown editor. ([Source Code](https://github.com/hackmdio/hackmd/)) `MIT` `JavaScript`
 
 
 ## Office Suites