Update README; beta release (#15098)

README.md

 # <img src="https://flutter.io/images/flutter-mark-square-100.png" alt="Flutter" width="40" height="40" /> Flutter [![Join Gitter Chat Channel -](https://badges.gitter.im/flutter/flutter.svg)](https://gitter.im/flutter/flutter?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![Build Status - Travis](https://travis-ci.org/flutter/flutter.svg?branch=master)](https://travis-ci.org/flutter/flutter) [![Build Status - AppVeyor](https://ci.appveyor.com/api/projects/status/meyi9evcny64a2mg/branch/master?svg=true)](https://ci.appveyor.com/project/flutter/flutter/branch/master) [![Coverage Status -](https://coveralls.io/repos/github/flutter/flutter/badge.svg?branch=master)](https://coveralls.io/github/flutter/flutter?branch=master)
 
-A new mobile app SDK to help developers and designers build modern mobile apps for iOS and Android. Flutter is an alpha, open-source project.
+A new mobile app SDK to help developers and designers build modern mobile apps for iOS and Android. Flutter is an open-source project currently in beta.
 
 ### Documentation