Add an section explaining the benefits of using package:ok_http. (dart-lang#1252)

Author: brianquinlan

pkgs/ok_http/README.md

@@ -4,6 +4,25 @@
 An Android Flutter plugin that provides access to the
 [OkHttp][] HTTP client.
 
+## Why use `package:ok_http`?
+
+### 👍 Increased compatibility and reduced disk profile
+
+`package:ok_http` is smaller and works on more devices than other packages.
+
+This size of the [example application][] APK file using different packages:
+
+| Package | APK Size (MiB) |
+|-|-|
+| **`ok_http`** | **20.3**  |
+| [`cronet_http`](https://pub.dev/packages/cronet_http) [^1] | 20.6 |
+| [`cronet_http` (embedded)](https://pub.dev/packages/cronet_http#use-embedded-cronet) [^2] | 34.4 |
+| `dart:io` [^3] | 20.4 |
+
+[^1]: Requires [Google Play Services][], which are not available on all devices.
+[^2]: Embeds the Cronet HTTP library.
+[^3]: Accessed through [`IOClient`](https://pub.dev/documentation/http/latest/io_client/IOClient-class.html).
+
 ## Status: experimental
 
 **NOTE**: This package is currently experimental and published under the
@@ -19,4 +38,6 @@ Your feedback is valuable and will help us evolve this package. For general
 feedback, suggestions, and comments, please file an issue in the
 [bug tracker](https://github.com/dart-lang/http/issues).
 
+[example application]: https://github.com/dart-lang/http/tree/master/pkgs/flutter_http_example
 [OkHttp]: https://square.github.io/okhttp/
+[Google Play Services]: https://developers.google.com/android/guides/overview