Update AdSupportIOS.js

Summary:
Looks like RN unlinked the RCTAdSupport module from the React project because it triggers some automatic detection from Apple. (from this thread facebook#1303). So this includes the instructions to manually add RCTAdSupport files from node_modules.

See: facebook#12623
Closes facebook#12657

Differential Revision: D4962321

Pulled By: hramos

fbshipit-source-id: ed9aed3b4c16e72d1f9557f5c16fcdadfdd743ea

Libraries/AdSupport/AdSupportIOS.js

@@ -13,6 +13,20 @@
 
 var AdSupport = require('NativeModules').AdSupport;
 
+/**
+ * `AdSupport` provides access to the "advertising identifier". If you link this library
+ * in your project, you may need to justify your use for this identifier when submitting
+ * your application to the App Store.
+ *
+ * In order to use `AdSupport` in your project, you must link the `RCTAdSupport` library.
+ * In Xcode, you can manually add the `RCTAdSupport.m` and `RCTAdSupport.h` files from
+ * `node_modules/react-native/Libraries/AdSupport/` to the `Libraries/React/Base/` folder
+ * of your current project.
+ *
+ * You can refer to [Linking](docs/linking-libraries-ios.html) for help.
+ *
+ */
+
 module.exports = {
   getAdvertisingId: function(onSuccess: Function, onFailure: Function) {
     AdSupport.getAdvertisingId(onSuccess, onFailure);