the postMessage API now works for getter methods

README.md

@@ -1065,18 +1065,38 @@ The framework has a built-in postMessage API that can be used when communicating
 <window>.postMessage( JSON.stringify({ method: 'slide', args: [ 2 ] }), '*' );
 ```
 
+#### postMessage Events
+
 When reveal.js runs inside of an iframe it can optionally bubble all of its events to the parent. Bubbled events are stringified JSON with three fields: namespace, eventName and state. Here's how you subscribe to them from the parent window:
 
 ```javascript
 window.addEventListener( 'message', function( event ) {
 	var data = JSON.parse( event.data );
-	if( data.namespace === 'reveal' && data.eventName ==='slidechanged' ) {
+	if( data.namespace === 'reveal' && data.eventName === 'slidechanged' ) {
 		// Slide changed, see data.state for slide number
 	}
 } );
 ```
 
-This cross-window messaging can be toggled on or off using configuration flags.
+#### postMessage Callbacks
+
+When you call any method via the postMessage API, reveal.js will dispatch a message with the return value. This is done so that you can call a getter method and see what the result is. Check out this example:
+
+```javascript
+<revealWindow>.postMessage( JSON.stringify({ method: 'getTotalSlides' }), '*' );
+
+window.addEventListener( 'message', function( event ) {
+	var data = JSON.parse( event.data );
+	// `data.method`` is the method that we invoked
+	if( data.namespace === 'reveal' && data.eventName === 'callback' && data.method === 'getTotalSlides' ) {
+		data.result // = the total number of slides
+	}
+} );
+```
+
+#### Turning postMessage on/off
+
+This cross-window messaging can be toggled on or off using configuration flags. These are the default values.
 
 ```javascript
 Reveal.initialize({