doc: add docs for batch micro benchmarks (#5595)

* docs: add docs for batch micro benchmarks

* typo

src/batch/benches/README.md

@@ -0,0 +1,34 @@
+# Micro Benchmark for Batch Executors
+
+We use [criterion](https://bheisler.github.io/criterion.rs/book/index.html) micro-benchmarking tool. For more details on how to write and run benchmarks, please refer to its documentation.
+
+## Run Benchmark
+
+Run all benchmarks
+
+```bash
+cargo bench -p risingwave_batch
+```
+
+Run a specific benchmark
+
+```bash
+cargo bench -p risingwave_batch -- <filter>
+```
+
+where `<filter>` is a regular expression matching the benchmark ID, e.g., 
+`top_n.rs` uses `BenchmarkId::new("TopNExecutor", params)` , so we can run TopN benchmarks with
+
+```bash
+# All TopN benchmarks
+cargo bench -p risingwave_batch -- TopN
+# One specific setting of TopN benchmarks
+cargo bench -p risingwave_batch -- "TopNExecutor/2048\(single_column: true\)"
+```
+
+> *Note*: `-p risingwave_batch` can be omitted if you are in the `src/batch` directory.
+
+## Add new Benchmarks
+
+* Add benchmark target to `src/batch/Cargo.toml`
+* Implement benchmarks in `src/batch/benches`, referring to existing ones