add readme for complex_layout benchmark (#5948)

dev/benchmarks/complex_layout/README.md

+# complex_layout
+
+## Scrolling benchmark
+
+To run the scrolling benchmark on a device:
+
+```
+flutter drive --profile test_driver/scroll_perf.dart
+```
+
+Results should be in the file `build/complex_layout_scroll_perf.timeline_summary.json`.
+
+More detailed logs should be in `build/complex_layout_scroll_perf.timeline.json`.
+
+
+## Startup benchmark
+
+To measure startup time on a device:
+
+```
+flutter run --profile --trace-startup
+```
+
+Results should be in the logs.
+
+Additional results should be in the file `build/start_up_info.json`.