say what the engine is (flutter#3029)

* say what the engine is

closes flutter/flutter#289

* Update README.md

README.md

@@ -6,13 +6,17 @@ 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 an early-stage open-source project._ We are still missing core
-features like accessibility, text input, localization, and more. However,
+The Flutter Engine is the runtime environment for hosting Flutter applications.
+Most developers will interact with Flutter via the Flutter Framework and APIs,
+which run inside the engine.
+
+_Flutter is an early-stage open-source project._ We are still adding
+features. However,
 you can build demos and examples today. We hope you try it out and send
 us [feedback](mailto:[email protected]).
 
  - For information about using Flutter to build apps, please see
-   the [getting started guide](http://flutter.io/getting-started/).
+   the [getting started guide](https://flutter.io/getting-started/).
 
  - For information about contributing to the Flutter framework, please see
    [the main Flutter repository](https://github.com/flutter/flutter/blob/master/CONTRIBUTING.md).