Documentation improvement.

Author: nnethercote

collector/benchmarks/README.md

@@ -194,6 +194,8 @@ Rust code being written today.
       --id Test --profiles=Check --scenarios=IncrPatched
       --include=$NEW_BENCHMARK`
   - Add the new entry to `collector/benchmarks/README.md`.
+  - `git add` the `Cargo.lock` file, if it's not already part of the
+    benchmark's committed code.
 - Consider the benchmarking time for the benchmark.
   - First, measure the entire compilation time with something like this, by
     doing this within the benchmark directory is good: