Restore the JS Spinning Cube demo

examples/js/README.md

 JavaScript Mojo Example Applications
 =====================
 
 hello.js, world.js - A minimal application that connects to another.
 
 wget.js - Uses the network service to load a URL.
 
+cube.js - A JS version of examples/sample_app.
 
 --- Running Mojo Applications --- 
 
 A Mojo application written in JavaScript is launched with mojo_shell like this:
 
   mojo_shell <js-application-url>
 
-Where js-application-url is either a file or an http URL that names a JS source
-file. The JS file itself must begin with a Mojo "shebang" that specifies the
-Mojo URL of the JS content handler. In other words, the first line of the JS
-source file must be:
+Where js-application-url is a URL understood by the shell. For example
+a file or an http URL that names a JS source file. The JS file itself
+must begin with a Mojo "shebang" that specifies the Mojo URL of the JS
+content handler. In other words, the first line of the JS source file
+must be:
 
   #!mojo:js_content_handler
 
 Following the shebang should be a single AMD module called "main" whose value
 is an Application subclass. The JS content handler will create an instance of
 the Application and make it the client of the Mojo shell. The JS content handler
 is itself a Mojo application and it's responsible for creating an instance of V8
 and loading the "main" JS module and all of the modules the main module
 depends on. 
 
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 response parameter. In the EchoString case that would be something like 
 {value: "foo"}.
 
 Similarly, the implementation of a Mojo interface functions that specify a
 response, must return a Promise. The implementation of EchoString() could
 be written like this:
 
   MyEchoStringImpl.prototype.EchoString = function(s) {
     return Promise.resolve({value: s});
   };