[realworld uses] Add BNF Playground

Author: marijnh

doc/realworld.html

@@ -34,6 +34,7 @@ <h2>CodeMirror real-world uses</h2>
       <li><a href="http://blog.bitbucket.org/2013/05/14/edit-your-code-in-the-cloud-with-bitbucket/">Bitbucket</a> (code hosting)</li>
       <li><a href="https://blogger.googleblog.com/2013/04/improvements-to-blogger-template-html.html">Blogger's theme editor</a></li>
       <li><a href="http://bluegriffon.org/">BlueGriffon</a> (HTML editor)</li>
+      <li><a href="https://bnfplayground.pauliankline.com/">BNF Playground</a> (grammar workbench)</li>
       <li><a href="https://github.com/isdampe/BosonEditorExperimental">Boson Editor</a> (code editor)</li>
       <li><a href="http://cargocollective.com/">Cargo Collective</a> (creative publishing platform)</li>
       <li><a href="https://developers.google.com/chrome-developer-tools/">Chrome DevTools</a></li>