Add cras_bench instructions

BUG=b:294176591
TEST=follow the readme

Change-Id: Ie0d64d7f762f1bec1d2ba54e6c7431f01094fde3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/5082325
Reviewed-by: Chih-Yang Hsia <paulhsia@chromium.org>
Tested-by: chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com <chromeos-cop-builder@chromeos-cop.iam.gserviceaccount.com>
Commit-Queue: Chih-Yang Hsia <paulhsia@chromium.org>

cras/benchmark/README.md

@@ -0,0 +1,49 @@
+# CRAS Benchmark
+
+## Build instructions
+
+1.  Install dependencies:
+
+    1.  Bazelisk: https://github.com/bazelbuild/bazelisk
+    1.  System dependencies:
+        1.  alsa
+        1.  libevent
+        1.  protobuf
+
+1.  Clone this repository and the webrtc-apm repository, so they are adjacent to each other.
+
+    ```
+    git clone https://chromium.googlesource.com/chromiumos/third_party/adhd
+    git clone https://chromium.googlesource.com/chromiumos/third_party/webrtc-apm
+    ```
+
+1.  Build `cras_bench` in the `adhd` directory:
+
+    ```
+    cd adhd
+    bazelisk build //cras/benchmark:cras_bench --//:apm -c opt --action_env=CC=clang --action_env=CXX=clang++ --copt=-march=x86-64-v3
+    ```
+
+    Notable options:
+
+    *   `-c opt` - enables optimized builds
+    *   `--action_env=CC=clang` `--action_env=CXX=clang++` - sets the C and C++ compilers to clang
+    *   `--copt` - sets C/C++ compiler options. You can add more with more `--copt=...`
+    *   `--copt=-march=x86-64-v3` - sets the target architecture. You should use the best one that fits your system
+
+    The resulting binary will be in `bazel-bin/cras/benchmark/cras_bench`.
+
+## Run instructions
+
+Run with
+
+```
+bazel-bin/cras/benchmark/cras_bench --benchmark_out=result.json
+```
+
+to get a JSON file with the results at `result.json`.
+
+## Editor note
+
+Run `bazel cquery --//:apm 'filter("^@pkg_config", deps(//cras/benchmark:cras_bench))'`
+to query up-to-date system dependencies.