Some documentation for running "flutter refresh" in the Gradle sample (#3201)

examples/hello_android/README.md

diff --git a/examples/hello_android/README.md b/examples/hello_android/README.md
index c7d64c4..d57c79d 100644
--- a/examples/hello_android/README.md
+++ b/examples/hello_android/README.md
@@ -14,3 +14,24 @@
 
 * `gradle wrapper`
 * `./gradlew build`
+
+## Updating the Dart code
+
+You can push new Dart code to a Flutter app during development without performing
+a full rebuild of the Android app package.
+
+The `flutter refresh` tool manages this process.  `flutter refresh` will build
+a snapshot of an app's Dart code, copy it to an Android device, and send an
+intent instructing the Android app to load the snapshot.
+
+To try this out:
+
+* Install and run this app on your device
+* Edit the Dart code in `app/src/flutter/lib`
+* cd `app/src/flutter`
+* `flutter refresh --activity com.example.flutter/.ExampleActivity`
+
+`flutter refresh` sends an `ACTION_RUN` intent with an extra containing the
+device filesystem path where the snapshot was copied.  `ExampleActivity.java`
+shows how an activity can handle this intent and load the new snapshot into
+a Flutter view.