Update README.md (#783)

adds documentation clarifying additional iOS setup for deep link tracking.

Author: alanjcharles

packages/plugins/plugin-appsflyer/README.md

@@ -41,6 +41,21 @@ const plugin = new AppsflyerPlugin();
 segmentClient.add({ plugin });
 ```
 
+### Tracking Deep Links on iOS
+
+The Analytics React Native SDK [requires additonal setup](https://github.com/segmentio/analytics-react-native#ios-deep-link-tracking-setup) to automatically track deep links. If you are also tracking [Universal Links](https://dev.appsflyer.com/hc/docs/ios-sdk-reference-appsflyerlib#continue), add the following to your `AppDelegate.m` : 
+
+```objc
+- (BOOL)application:(UIApplication *)application continueUserActivity:(nonnull NSUserActivity *)userActivity
+ restorationHandler:(nonnull void (^)(NSArray<id<UIUserActivityRestoring>> * _Nullable))restorationHandler
+{
+  if ([userActivity.activityType isEqualToString: NSUserActivityTypeBrowsingWeb]) {
+    NSURL *url = userActivity.webpageURL;
+    NSDictionary *options = @{};
+    [AnalyticsReactNative trackDeepLink:url withOptions:options];
+  }
+ ```
+
 ## Support
 
 Please use Github issues, Pull Requests, or feel free to reach out to our [support team](https://segment.com/help/).
@@ -72,4 +87,4 @@ AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
-```
+```