Added new blocks editor issue to troubleshooting page

appinventor/docs/learn/troubleshooting.html

@@ -139,6 +139,23 @@
         </a>
         .
        </p>
+       <b>
+        The Blocks Editor won't open: The application starts
+       downloading and then nothing happens.
+       </b>
+       <p>
+       To diagnose this, first verify that your computer can run the
+               Java configuration tests and that you can load and run the
+               Notepad test.  If not, you'll need to check your Java and Java
+               Web Start configurations.   If Notepad runs, but the
+               Blocks Editor won't open, then this is most likely the
+               result of a firewall or other blocking software that is
+               preventing the blocks editor from communicating with
+               the browser, so check for firewalls and browser
+               security blocking extensions.   There have also been
+               reports of cases where this behavior has been caused by
+               misconfigured virus protection programs.
+       </p>
        <b>
         I'm having various Java problems, and I'm running on Linux and OpenJDK.
        </b>