instructions for building the gallery (#4979)

* instructions for building the gallery

* tweaks from review

* tweaks from review

examples/flutter_gallery/README.md

 # Flutter gallery
 
-Demo app for the Material Design widgets and other features provided by Flutter.
+Demo app for the material design widgets and other features provided by Flutter.
+
+## Building
+
+You can follow these instructions to build the gallery app
+and install it onto your device.
+
+### Prereqs
+
+If you are new to Flutter, please first follow
+the [Flutter Setup](https://flutter.io/setup/) guide.
+
+### Building and installing the Flutter app
+
+* `cd $FLUTTER_ROOT/examples/flutter_gallery`
+* `flutter upgrade`
+* `flutter -d <device name> run --release`
+
+The `flutter run --release` command both builds and installed the Flutter app.
 
 ## Icon