[Benchmarking] Doc for running unit tests.

This addresses concerns that came up in
https://chromium-review.googlesource.com/c/chromium/src/+/1913287/3#message-a8c363497a19a03f9c10079fa5fca6ed67263935

R=abennetts@google.com, perezju@chromium.org

Bug: 949246
Change-Id: I2686272fcfafaafb71b20a5024ec99f2e30e9885
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925592
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Auto-Submit: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718372}

tools/perf/README.md

@@ -7,7 +7,7 @@
 
 This directory contains a variety of command line tools that can be used to run
 benchmarks, interact with speed services, and manage performance waterfall
-configurations.
+configurations. It also has commands for running functional unittests.
 
 Note you can also read the higher level [Chrome Speed][speed] documentation to
 learn more about the team organization and, in particular, the top level view
@@ -16,6 +16,19 @@ of [How Chrome Measures Performance][chrome_perf_how].
 [speed]: /docs/speed/README.md
 [chrome_perf_how]: /docs/speed/how_does_chrome_measure_performance.md
 
+## run_tests
+
+This command allows you to run functional tests against the python code in this
+directory. For example, try:
+
+```
+./run_tests results_dashboard_unittest
+```
+
+Note that the positional argument can be any substring within the test name.
+
+This may require you to set up your `gsutil config` first.
+
 ## run_benchmark
 
 This command allows running benchmarks defined in the chromium repository,