Update CONTRIBUTING.md (#1391)

CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 0e390e8..f50d753 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -48,6 +48,10 @@
 
 - `flutter run -d chrome`
 
+To test release performance:
+
+- `flutter run -d web-server --release --dart-define=FLUTTER_WEB_USE_SKIA=true`
+
 You can also use `-d headless-server`, which will start a headless server that serves the HTML
 files for the DevTools Flutter app.