updated README.

Author: jestoniyap

README.md

@@ -382,7 +382,7 @@ branchUniversalObj.showShareSheet({
 });
 ```
 
-##### Share Sheet Callbacks
+##### Share Sheet Callbacks (Android ONLY)
 
 To implement the callback, you must add listeners to the following events:
 
@@ -426,6 +426,10 @@ branchUniversalObj.onChannelSelected(function (res) {
 });
 ```
 
+**Note:** Callbacks in iOS are ignored. There is no need to implement them as the events are handled by `UIActivityViewController`.
+
+**Note:** Avoid passing `alias` in iOS. Adding an `alias` key in the `options` parameter will return a Non-Universal link which will not work in iOS 9.2.
+
 ### <a id="listOnSpotlight"></a>listOnSpotlight()
 
 **Note: iOS only.** Used for Spotlight listing