adding note regarding the pip mode on iOS

API.md

@@ -667,6 +667,8 @@ Determine whether the media should played as picture in picture.
 * **false (default)** - Don't not play as picture in picture
 * **true** - Play the media as picture in picture
 
+NOTE: Video ads cannot start when you are using the PIP on iOS (more info available at [Google IMA SDK Docs](https://developers.google.com/interactive-media-ads/docs/sdks/ios/client-side/picture_in_picture?hl=en#starting_ads)). If you are using custom controls, you must disable your PIP button when you receive the ```STARTED``` event from ```onReceiveAdEvent``` and show it again when you receive the ```ALL_ADS_COMPLETED``` event.
+
 Platforms: iOS
 
 #### playInBackground