add extra config for idfa

Author: Niall Brennan

src/connections/sources/catalog/libraries/mobile/react-native/classic.md

@@ -506,6 +506,16 @@ import analytics from '@segment/analytics-react-native';
 analytics.setIDFA("123");
 ```
 
+You will also need to set the `trackAdvertising` property to true in your configuration.
+
+```java
+analytics.setup('YOUR_WRITE_KEY', {
+  // ...
+  ios:{
+    trackAdvertising: true
+  },
+})
+```
 
 ### Using a custom anonymousID