[web_embedding] Update README to link to our first community contribs. (#1980)

Update README.md of the `web_embedding` sample to include links to
community contributions.

* Follow up of: https://github.com/flutter/samples/pull/1945


## 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

---------

Co-authored-by: Brett Morgan <brett.morgan@gmail.com>

web_embedding/README.md

@@ -9,3 +9,25 @@ This directory contains examples of how to embed Flutter in web apps (without if
 
 Check the `README.md` of each example for more details on how to run it, and the
 "Points of Interest" it may contain.
+
+## Community Contributions
+
+Members of the community have graciously ported and contributed the following examples
+of Flutter Web embedding into other web frameworks:
+
+| Author | Host Framework | Code |
+|--------|----------------|------|
+| [@p-mazhnik][] | **React JS** | [p-mazhnik/flutter-embedding cra-flutter][] |
+| [@p-mazhnik][] | **React Native** | [p-mazhnik/flutter-embedding expo-flutter][] |
+
+
+_(All contributions are welcome! Please, [create an issue][] or open a PR to let us know
+how you've embedded a Flutter Web app with your favorite web framework_
+(Vue? Svelte? Ember? Aurelia? jQuery? MooTools? Prototype?), _so it can be added to the table above!)_
+
+  [@p-mazhnik]: https://github.com/p-mazhnik
+  [p-mazhnik/flutter-embedding cra-flutter]: https://github.com/p-mazhnik/flutter-embedding/tree/main/cra-flutter
+  [p-mazhnik/flutter-embedding expo-flutter]: https://github.com/p-mazhnik/flutter-embedding/tree/main/expo-flutter
+  [create an issue]: https://github.com/flutter/samples/issues/new
+  
+