Update description on docs (#14660)

dev/docs/README.md

-Flutter is a new way to build high-performance, cross-platform mobile apps.
-Flutter is optimized for today's, and tomorrow's, mobile devices. We are focused
-on low-latency input and high frame rates on Android and iOS.
+Flutter is Google’s mobile UI framework to craft high-quality native interfaces.
+Integrates with iOS and Android, works with existing code,
+and is free and open source.
 
 See the [getting started guide](https://flutter.io/getting-started/) for
 information about using Flutter.