Create README.md (#1823)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[CLA]: https://cla.developers.google.com/
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md

next_gen_ui_demo/README.md

@@ -0,0 +1,9 @@
+This is a demo version of the [Building next generation UIs in Flutter][] codelab.
+
+This sample takes code from each step of the codelab linked above and turns it into
+a single application with forward and backward buttons to enable stepping through the
+steps of the codelab.
+
+![Next Generation UI demo screenshot](next_gen_ui_demo.png)
+
+  [Building next generation UIs in Flutter]: https://codelabs.developers.google.com/codelabs/flutter-next-gen-uis