benchmark: Create README.md

When trying to run the benchmark I found little documentation on how to
run the benchmark.

tools/benchmark/README.md

@@ -0,0 +1,9 @@
+# etcd/tools/benchmark
+
+`etcd/tools/benchmark` is the official benchmarking tool for etcd clusters.
+
+## Install
+
+```bash
+go get github.com/coreos/etcd/tools/benchmark
+```