add readme with instructions how to add a new benchmark file

benchmarks/README.md

@@ -0,0 +1,7 @@
+# `datadog` Benchmarks
+
+## Adding a New Benchmark File
+
+- Add the new file to `run_all.sh` in this directory
+- Add the new file to `spec/datadog/tracing/validate_benchmarks_spec.rb`
+which validates that the benchmarks are error-free in the test suite.