Update README.md for consistency with framework (#9428)

Updates our README.md to use consistent wording with that of the
framework.

Mostly though, this is intended to trigger a build that verifies that
we're correctly building Fuchsia build products on Linux and Mac LUCI
infrastructure.

See: https://chromium-review.googlesource.com/c/chromium/tools/build/+/1671747

README.md

 Flutter Engine
 ==============
-[![Build Status](https://api.cirrus-ci.com/github/flutter/engine.svg)](https://cirrus-ci.com/github/flutter/engine)
+[![Build Status - Cirrus][]][Build status]
 
-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 app SDK for crafting high-quality native interfaces
+on iOS and Android in record time. Flutter works with existing code, is used by
+developers and organizations around the world, and is free and open source.
 
 The Flutter Engine is a portable runtime for hosting
 [Flutter](https://flutter.dev) applications.  It implements Flutter's core
@@ -20,7 +20,9 @@ site at [Flutter.dev](https://flutter.dev). For specific information
 about Flutter's APIs, consider our API reference which can be found at
 the [docs.flutter.dev](https://docs.flutter.dev/).
 
-If you intend to contribute to Flutter, welcome! You are encouraged to
-start with [our contributor
-guide](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md),
-which helps onboard new team members.
+Flutter is a fully open source project, and we welcome contributions.
+Information on how to get started can be found at our
+[contributor guide](CONTRIBUTING.md).
+
+[Build Status - Cirrus]: https://api.cirrus-ci.com/github/flutter/engine.svg
+[Build status]: https://cirrus-ci.com/github/flutter/engine