doc(example): adds comments to the hello world app.

Explains the basic concepts as seen in the demo app, but it is not a
substitute for proper documentation which will come later.

modules/examples/src/hello_world/app.js

@@ -1,10 +1,30 @@
 import {bootstrap, Component, Decorator, TemplateConfig, NgElement} from 'core/core';
 
+// Angular 2.0 supports 3 basic types of directives:
+// - Component - the basic building blocks of Angular 2.0 apps. Backed by
+//   ShadowDom.(http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom/)
+// - Decorator - add behavior to existing elements.
+// - Template - allow for stamping out of a html template (not in this demo).
+
+// @Component is AtScript syntax to annotate the HelloCmp class as an Angular
+// 2.0 component.
 @Component({
+  // The Selector prop tells Angular on which elements to instantiate this
+  // class. The syntax supported is a basic subset of CSS selectors, for example
+  // 'element', '[attr]', [attr=foo]', etc.
   selector: 'hello-app',
+  // These are services that would be created if a class in the component's
+  // template tries to inject them.
   componentServices: [GreetingService],
   template: new TemplateConfig({
+    // The template for the component.
+    // Expressions in the template (like {{greeting}}) are evaluated in the
+    // context of the HelloCmp class below.
     inline: `{{greeting}} <span red>world</foo>!`,
+    // All directives used in the template need to be specified. This allows for
+    // modularity (RedDec can only be used in this template)
+    // and better tooling (the template can be invalidated if the attribute is
+    // misspelled).
     directives: [RedDec]
   })
 })
@@ -14,15 +34,20 @@ class HelloCmp {
   }
 }
 
+// Decorators are light-weight. They don't allow for templates, or new
+// expression contexts (use @Component or @Template for those needs).
 @Decorator({
   selector: '[red]'
 })
 class RedDec {
+  // NgElement is always injectable and it wraps the element on which the
+  // directive was found by the compiler.
   constructor(el: NgElement) {
     el.domElement.style.color = 'red';
   }
 }
 
+// A service used by the HelloCmp component.
 class GreetingService {
   constructor() {
     this.greeting = 'hello';
@@ -30,5 +55,12 @@ class GreetingService {
 }
 
 export function main() {
+  // Bootstrapping only requires specifying a root component.
+  // The boundary between the Angular application and the rest of the page is
+  // the shadowDom of this root component.
+  // The selector of the component passed in is used to find where to insert the
+  // application.
+  // You can use the light dom of the <hello-app> tag as temporary content (for
+  // example 'Loading...') before the application is ready.
   bootstrap(HelloCmp);
 }