Improve Flutter example readme (#425)

* Improve readme

* Update flutter/example/README.md

Co-authored-by: Manoel Aranda Neto <[email protected]>

* Update flutter/example/README.md

Co-authored-by: Bruno Garcia <[email protected]>

Co-authored-by: Manoel Aranda Neto <[email protected]>
Co-authored-by: Bruno Garcia <[email protected]>

Author: 3 people

flutter/example/README.md

@@ -1,16 +1,40 @@
-# sentry_flutter_example
+<p align="center">
+  <a href="https://sentry.io" target="_blank" align="center">
+    <img src="https://sentry-brand.storage.googleapis.com/sentry-logo-black.png" width="280">
+  </a>
+  <br/>
+</p>
 
-Demonstrates how to use the sentry_flutter plugin.
+Sentry Example for Flutter
+===========
 
-## Getting Started
+This is a Flutter app that crashes intentionally in various ways. It demonstrates
+how to capture errors and report them to [sentry.io](https://sentry.io/). 
 
-This project is a starting point for a Flutter application.
+# Running this sample
 
-A few resources to get you started if this is your first Flutter project:
+> First modify the `main.dart` and add your own DSN. You can get one at
+> [sentry.io](https://sentry.io/). This will make sure you can see the result of
+> running this app by looking at what this creates in Sentry.
 
-- [Lab: Write your first Flutter app](https://flutter.dev/docs/get-started/codelab)
-- [Cookbook: Useful Flutter samples](https://flutter.dev/docs/cookbook)
+You need to install [Flutter](https://flutter.dev/docs/get-started/install) in
+order to run this sample. Once you have Flutter installed you can run it via
+`flutter run`.
 
-For help getting started with Flutter, view our
-[online documentation](https://flutter.dev/docs), which offers tutorials,
-samples, guidance on mobile development, and a full API reference.
+You should see something like this:
+
+<img src="img/ios_simulator.png" />
+
+If you click in order on `Open another scaffold`, `Dart: web request` and 
+`Dart: try catch`, you should see the following image on
+[sentry.io](https://sentry.io/).
+
+Note the event shows only the application frames by default. You can click
+on the `Full` button to see the complete stack trace. There's also a list of breadcrumbs for
+that exception. In order to get HTTP and navigational breadcrumbs you need to
+use
+[`SentryHttpClient`](https://docs.sentry.io/platforms/dart/usage/advanced-usage/#automatic-breadcrumbs)
+and the
+[`SentryNavigatorObserver`](https://docs.sentry.io/platforms/flutter/enriching-events/breadcrumbs/#automatic-breadcrumbs).
+
+<img src="img/sentry_dashboard.png" />