Add a breadcrumb for people who want to run the web tests (#43865)

Author: Hixie

testing/run_tests.sh

@@ -6,3 +6,6 @@ BUILD_VARIANT="${1:-host_debug_unopt}"
 CURRENT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" >/dev/null 2>&1 && pwd )"
 
 python3 "${CURRENT_DIR}/run_tests.py" --variant="${BUILD_VARIANT}" --type=engine,dart,benchmarks
+
+# This does not run the web_ui tests. To run those, use:
+#   lib/web_ui/dev/felt test