Updated README to indicate that the IMA plugin must be initialized be…

…fore calling play(). (googleads#213)

README.md

@@ -59,13 +59,16 @@ var options = {
   adTagUrl: 'YOUR_AD_TAG'
 };
 
+// This must be called before player.play() below.
 player.ima(options);
 player.ima.requestAds();
 // On mobile devices, you must call initializeAdDisplayContainer as the result
 // of a user action (e.g. button click). If you do not make this call, the SDK
 // will make it for you, but not as the result of a user action. For more info
 // see our examples, all of which are set up to work on mobile devices.
 // player.ima.initializeAdDisplayContainer();
+
+// This must be called after player.ima(...) above.
 player.play();
 ```