readme manager initial setup (#264)

* readme manager initial setup

This PR makes the README ready to be managed by the readme manager. It adds custom comment tags to the readme, which are invisible when the README is rendered, but tell the readme manager where to insert and update content.

* Update README.md

README.md

@@ -1,8 +1,12 @@
+<!--BEGIN_BANNER_IMAGE--><!--END_BANNER_IMAGE-->
+
 [![pub package](https://img.shields.io/pub/v/livekit_client?label=livekit_client&color=blue)](https://pub.dev/packages/livekit_client)
 
 # LiveKit Flutter SDK
 
+<!--BEGIN_DESCRIPTION-->
 Official Flutter SDK for [LiveKit](https://livekit.io). Easily add real-time video and audio to your Flutter apps.
+<!--END_DESCRIPTION-->
 
 This package is published to pub.dev as [livekit_client](https://pub.dev/packages/livekit_client).
 
@@ -408,3 +412,5 @@ Apache License 2.0
 ## Thanks
 
 A huge thank you to [flutter-webrtc](https://github.com/flutter-webrtc/flutter-webrtc) for making it possible to use WebRTC in Flutter.
+
+<!--BEGIN_REPO_NAV--><!--END_REPO_NAV-->