Added HackMD in the real-world uses doc

Author: jackycute

doc/realworld.html

@@ -85,6 +85,7 @@ <h2>CodeMirror real-world uses</h2>
       <li><a href="https://github.com/github/android">GitHub's Android app</a></li>
       <li><a href="https://script.google.com/">Google Apps Script</a></li>
       <li><a href="http://web.uvic.ca/~siefkenj/graphit/graphit.html">Graphit</a> (function graphing)</li>
+      <li><a href="https://hackmd.io">HackMD</a> (Realtime collaborative markdown notes on all platforms)</li>
       <li><a href="http://www.handcraft.com/">Handcraft</a> (HTML prototyping)</li>
       <li><a href="http://hawkee.com/">Hawkee</a></li>
       <li><a href="http://try.haxe.org">Haxe</a> (Haxe Playground) </li>