Add benchmarks/README.md.
R=etiennej@chromium.org
Review URL: https://codereview.chromium.org/1412863005 .
diff --git a/benchmarks/README.md b/benchmarks/README.md
new file mode 100644
index 0000000..a090bd5
--- /dev/null
+++ b/benchmarks/README.md
@@ -0,0 +1,13 @@
+# benchmarks
+
+This directory contains mojo applications that are run as performance benchmarks
+by [mojo_benchmark](../mojo/devtools/common/mojo_benchmark).
+
+See also:
+ - [documentation](../mojo/devtools/common/docs/mojo_benchmark.md) for
+ `mojo_benchmark`
+
+## Legacy benchmarks
+`benchmark_runner.py` and the `startup` benchmark are the remainings of the
+previous take on performance benchmarks. These can be dropped once we can track
+startup time of the shell itself using the new system.
