Add an example for FlutterError.onError and runZoned (#41)

Author: filiph

README.md

@@ -47,10 +47,48 @@ main() async {
 
 ## Tips for catching errors
 
-- use a `try/catch` block
-- create a `Zone` with an error handler, e.g. using [runZoned][run_zoned]
-- in Flutter, use [FlutterError.onError][flutter_error]
-- use `Isolate.current.addErrorListener` to capture uncaught errors in the root zone
+- Use a `try/catch` block, like in the example above.
+- Create a `Zone` with an error handler, e.g. using [runZoned][run_zoned].
+  
+  ```dart
+  var sentry = SentryClient(dsn: "https://...");
+  // Run the whole app in a zone to capture all uncaught errors.
+  runZoned(
+    () => runApp(MyApp()),
+    onError: (Object error, StackTrace stackTrace) {
+      try {
+        sentry.captureException(
+          exception: error,
+          stackTrace: stackTrace,
+        );
+        print('Error sent to sentry.io: $error');
+      } catch (e) {
+        print('Sending report to sentry.io failed: $e');
+        print('Original error: $error');
+      }
+    },
+  );
+  ```
+- For Flutter-specific errors (such as layout failures), use [FlutterError.onError][flutter_error]. For example:
+  
+  ```dart
+  var sentry = SentryClient(dsn: "https://...");
+  FlutterError.onError = (details, {bool forceReport = false}) {
+    try {
+      sentry.captureException(
+        exception: details.exception,
+        stackTrace: details.stack,
+      );
+    } catch (e) {
+      print('Sending report to sentry.io failed: $e');
+    } finally {
+      // Also use Flutter's pretty error logging to the device's console.
+      FlutterError.dumpErrorToConsole(details, forceReport: forceReport);
+    }
+  };
+  ```
+- Use `Isolate.current.addErrorListener` to capture uncaught errors 
+  in the root zone.
 
 [run_zoned]: https://api.dartlang.org/stable/dart-async/runZoned.html
 [flutter_error]: https://docs.flutter.io/flutter/foundation/FlutterError/onError.html