update readme file

Author: nturgut

e2etests/web/regular_integration_tests/README.md

@@ -37,3 +37,47 @@ flutter drive -v --target=test_driver/text_editing_integration.dart -d web-serve
 ```
 
 More details for "Running Flutter Driver tests with Web" can be found in [wiki](https://github.com/flutter/flutter/wiki/Running-Flutter-Driver-tests-with-Web).
+
+## Adding screenshot tests
+
+In order to test screenshot tests the tests on the driver side needs to call the `integration_test` package with an `onScreenshot` callback which can do a comparison between the `screenshotBytes` taken during the test and a golden file. We added a utility method that can do this comparison by using a golden in `flutter/goldens` repository. In order to use screenshot testing follow these steps:
+
+1. Call `screenshot_support.dart` from the driver side test (example: text_editing_integration_test.dart). Default value for `diffRateFailure` is 0.5 .
+
+```
+import 'package:regular_integration_tests/screenshot_support.dart' as test;
+
+Future<void> main() async {
+  final double kMaxDiffRateFailure = 0.1;
+  await test.runTestWithScreenshots(diffRateFailure = kMaxDiffRateFailure );
+}
+```
+
+2. In order to add the screenshot golden initially or to update the existing one, we need to set UPDATE_GOLDENS flag to environment.
+
+```
+export UPDATE_GOLDENS=true
+```
+
+3. Run the specific test or run all integration tests
+
+```
+flutter drive -v --target=test_driver/text_editing_integration.dart -d web-server --release --local-engine=host_debug_unopt
+```
+
+```
+felt test --integration-tests-only
+```
+
+4. The golden will be under `engine/src/flutter/lib/web_ui/.dart_tool/goldens/engine/web/` directory, you should create a PR for that file and merge it to `flutter/goldens`.
+
+5. Get the commit no and replace the `revision` in this file: `engine/src/flutter/lib/web_ui/dev/goldens_lock.yaml`
+
+6. Don't forget to rechange the flag to switch goldens from update mode to comparison mode.
+
+
+```
+export UPDATE_GOLDENS=false
+```
+
+7. Screenshot tests should work after this.