Added link for Edabit

Edabit is a free tool for practicing / learning to code.

Author: mattmacpherson

doc/realworld.html

@@ -70,6 +70,7 @@ <h2>CodeMirror real-world uses</h2>
       <li><a href="http://ireneros.com/deck/deck.js-codemirror/introduction/#textarea-code">Deck.js integration</a> (slides with editors)</li>
       <li><a href="http://www.dbninja.com">DbNinja</a> (MySQL access interface)</li>
       <li><a href="http://www.ecsspert.com/">eCSSpert</a> (CSS demos and experiments)</li>
+      <li><a href="https://edabit.com">Edabit</a> (coding challenges)</li>
       <li><a href="http://elm-lang.org/Examples.elm">Elm language examples</a></li>
       <li><a href="http://eloquentjavascript.net/chapter1.html">Eloquent JavaScript</a> (book)</li>
       <li><a href="http://emmet.io">Emmet</a> (fast XML editing)</li>