Add SageMathCloud to real world examples

doc/realworld.html

@@ -123,6 +123,7 @@
       <li><a href="http://www.quivive-file-manager.com">Quivive File Manager</a></li>
       <li><a href="http://rascalmicro.com/docs/basic-tutorial-getting-started.html">Rascal</a> (tiny computer)</li>
       <li><a href="https://www.realtime.io/">RealTime.io</a> (Internet-of-Things infrastructure)</li>
+      <li><a href="https://cloud.sagemath.com/">SageMathCloud</a> (interactive mathematical software environment)</li>
       <li><a href="https://chrome.google.com/webstore/detail/servephp/mnpikomdchjhkhbhmbboehfdjkobbfpo">ServePHP</a> (PHP code testing in Chrome dev tools)</li>
       <li><a href="https://www.shadertoy.com/">Shadertoy</a> (shader sharing)</li>
       <li><a href="http://www.sketchpatch.net/labs/livecodelabIntro.html">sketchPatch Livecodelab</a></li>