Add hero image to README. (#428)

Author: jeroenleenarts

.readme-assets/Github-Graphic-Flutter.jpg

@@ -1,5 +1,7 @@
 # Official Flutter packages for the [Stream Video SDK](https://getstream.io/video/sdk/flutter/)
 
+<img src=".readme-assets/Github-Graphic-Flutter.jpg" alt="Stream Video for Flutter Header image" style="box-shadow: 0 3px 10px rgb(0 0 0 / 0.2); border-radius: 1rem" />
+
 ## Quick Links
 
 - [Register](https://getstream.io/video/) to get an API key for Stream Video