[tools/perf] Add README.md file

Start with a simple file adding pointers for further documentation of
`run_benchmark` command.

Follow up CLs can add on top by documenting other scripts in this
directory.

Bug: 922030
Change-Id: Ife3e22246746c416abf70ca0b0e92c2b51e2819d
Reviewed-on: https://chromium-review.googlesource.com/c/1489231Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635765}

tools/perf/README.md

+<!-- Copyright 2019 The Chromium Authors. All rights reserved.
+     Use of this source code is governed by a BSD-style license that can be
+     found in the LICENSE file.
+-->
+
+# Performance tools
+
+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.
+
+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
+of [How Chrome Measures Performance][chrome_perf].
+
+[speed]: /docs/speed/README.md
+[chrome_perf]: /docs/speed/how_does_chrome_measure_performance.md
+
+## run_benchmark
+
+This command allows running benchmarks defined in the chromium repository,
+specifically in [tools/perf/benchmarks][benchmarks_dir]. If you need it,
+documentation is available on how to [run benchmarks locally][run_locally]
+and how to properly [set up your device][device_setup].
+
+[benchmarks_dir]: https://cs.chromium.org/chromium/src/tools/perf/benchmarks/
+[run_locally]: https://chromium.googlesource.com/catapult.git/+/HEAD/telemetry/docs/run_benchmarks_locally.md
+[device_setup]: /docs/speed/benchmark/telemetry_device_setup.md
+
+
+## TODO
+
+Documentation for other commands in this directory is also coming.