Benchmarks: Add README

_benchmarks/README.md

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+# Benchmarks
+
+This folder contains code for running the [common Elasticsearch client benchmarks](https://clients-benchmarks.elastic.co).
+
+The `benchmarks` package contains the general configuration for the benchmarks. The `runner` package provides the code for running and measuring the client actions, storing the results in an Elasticsearch cluster. The individual client interactions are defined by the `actions` package.
+
+The code for setting up the benchmarks infrastructure is available at
+<https://github.com/elastic/elasticsearch-clients-benchmarks>. It uses Terraform to create, setup and configure the "target" and "runner" system.
+