Add debugging details to add_to_app sample README. (#2168)

Fixes flutter/flutter#143550

## 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

add_to_app/README.md

@@ -1,8 +1,8 @@
 # Add-to-App Samples
 
 This directory contains Android and iOS projects that import and use a Flutter
-module. They're designed to show recommended approaches for adding Flutter to
-existing Android and iOS apps.
+module. They're designed to show recommended approaches for [adding Flutter to
+existing Android and iOS apps](https://docs.flutter.dev/add-to-app).
 
 ## Samples Listing
 
@@ -44,6 +44,13 @@ sudo gem install cocoapods
 
 See https://guides.cocoapods.org/using/getting-started.html for more details.
 
+## Debugging
+
+You can `flutter attach` to the running host application to [debug the Flutter
+module](https://docs.flutter.dev/add-to-app/debugging). This will
+allow you to hot reload, set breakpoints, and use DevTools and other debugging
+functionality, similar to a full Flutter app.
+
 ## Questions/issues
 
 If you have a general question about incorporating Flutter into an existing