Add content resizing sample (#2756)

Add a content resizing sample for iOS to support
https://github.com/flutter/flutter/pull/177410

## 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 have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).

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

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

---------

Co-authored-by: Eric Windmill <eric@ericwindmill.com>

add_to_app/ios_content_resizing/README.md

@@ -0,0 +1,38 @@
+# ios_content_resizing
+
+Embeds an instance of Flutter into an existing iOS app that will dynamically resize itself based off of it's content. 
+
+## Description
+
+This sample showcases the integration of a self-resizing Flutter view within a native UIKit environment. The Flutter module features an expanding column of widgets. As items are added, the hosting UIView will automatically adjusts its [intrinsicContentSize](https://developer.apple.com/documentation/uikit/uiview/intrinsiccontentsize) to accommodate the new content size.
+
+## tl;dr
+
+If you're just looking to get up and running quickly, these bash commands will
+fetch packages and set up dependencies (note that the above commands assume
+you're building for both iOS and Android, with both toolchains installed):
+
+```bash
+  #!/bin/bash
+  set -e
+
+  cd flutter_module/
+  flutter pub get
+
+  # For iOS builds:
+  cd ../ios_content_resizing
+  pod install
+  open ios_content_resizing.xcworkspace
+  # Then, click "Run" in Xcode to launch the app into your Simulator or device
+```
+
+## Requirements
+
+* Flutter
+* iOS
+  * Xcode
+  * Cocoapods
+
+## Questions/issues
+
+See [add_to_app/README.md](../README.md) for further help.