Update docs for benchmarks (#5999)

We changed how `flutter run --release` works (it quits without showing
logs now, since it can't tell when the app closes).

dev/benchmarks/microbenchmarks/README.md

 # microbenchmarks
 
-To run these benchmarks on a device:
+To run these benchmarks on a device, first run `flutter logs' in one
+window to see the device logs, then, in a different window, run any of
+these:
 
 ```
 flutter run --release lib/gestures/velocity_tracker_data.dart