Merge pull request #3346 from segmentio/niall/rn_ids

Add info for device identifiers

Author: pwseg

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

@@ -500,6 +500,11 @@ Segment supports a large number of [Cloud-mode](/docs/connections/destinations/#
 - [Facebook App Events](https://www.npmjs.com/package/@segment/analytics-react-native-plugin-facebook-app-events){:target="_blank"}
 - [Firebase](https://www.npmjs.com/package/@segment/analytics-react-native-plugin-firebase){:target="_blank"}
 
+## Device identifiers
+On Android, Segment's React Native library generates a unique ID by using the DRM API as context.device.id. Some destinations rely on this field being the Android ID, so be sure to double-check the destination’s vendor documentation. If you choose to override the default value using a plugin, make sure the identifier you choose complies with Google’s User Data Policy. For iOS the context.device.id is set the IDFV.
+
+To collect the Android Advertising ID provided by Play Services, Segment provides a [plugin](https://github.com/segmentio/analytics-react-native/tree/master/packages/plugins/plugin-advertising-id){:target="_blank"} that can be used to collect that value. This value is set to context.device.advertisingId. For iOS, this [plugin](https://github.com/segmentio/analytics-react-native/tree/master/packages/plugins/plugin-idfa){:target="_blank"} can be used to set the IDFA context.device.advertisingId property.
+
 ## FAQs
 ### Can I use the catalog of device-mode destinations from Segment's 1.X.X React-Native release?
 No, only the plugins listed above are supported in device-mode for Analytics React Native 2.0.