add integration tests and update README

example/compare_goldens.sh

@@ -0,0 +1,9 @@
+#!/bin/bash
+device=$1
+if [ -z "$device" ]; then
+    echo "Usage: $0 <device_id>"
+    exit 1;
+fi
+
+rm integration_test/goldens/{ios,macos,windows,android}/diffs/*.png
+flutter drive --driver=test_driver/integration_test.dart  -d $1 --target=integration_test/plugin_integration_test.dart