chore: add README to bench_util/ (#10712)

bench_util/README.md

@@ -0,0 +1,29 @@
+# Benching utility for `deno_core` op system
+
+Example:
+
+```rust
+use deno_bench_util::bench_or_profile;
+use deno_bench_util::bencher::{benchmark_group, Bencher};
+use deno_bench_util::bench_js_sync};
+
+use deno_core::op_sync;
+use deno_core::serialize_op_result;
+use deno_core::JsRuntime;
+use deno_core::Op;
+use deno_core::OpState;
+
+fn setup(runtime: &mut JsRuntime) {
+  runtime.register_op("nop", |state, _| {
+    Op::Sync(serialize_op_result(Ok(9), state))
+  });
+  runtime.sync_ops_cache();
+}
+
+fn bench_op_nop(b: &mut Bencher) {
+  bench_js_sync(b, r#"Deno.core.opSync("nop", null, null, null);"#, setup);
+}
+
+benchmark_group!(benches, bench_op_nop);
+bench_or_profile!(benches);
+```