Update instructions for running benchmarks

packages/unit/benchmark/README.md

-Benchmarks
-==========
+# Benchmarks
+
 This directory (and its sub-directories) contain benchmarks for Flutter.
 The reporting format for benchmarks is not standardized yet, so benchmarks
-here are typically run by hand. To run a particular benchmark, use a command
-similar to that used to run individual unit tests. For example:
+here are typically run by hand. To run a benchmark:
+
+1. Build `sky_shell` for Linux Release using the instructions in the
+   [Engine repository](https://github.com/flutter/engine).
+
+2. Run `pub get` in the `packages/unit` directory.
+
+3. Run the benchmark:
 
 ```
-sky/tools/run_tests --debug -r expanded benchmark/gestures/velocity_tracker_bench.dart
+/path/to/engine/src/out/Release/sky_shell packages/unit/benchmark/stocks/layout_bench.dart --package-root=packages/unit/package
 ```
-
-(The `-r expanded` flag prints one line per test, which can be more helpful
-than the default format when running individual tests.)