Add benchmark suite readme

rbx_binary/benches/README.md

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+# `rbx_binary` Benchmark Suite
+
+This directory contains a suite of benchmarks used to measure the performance of the `rbx_binary` serializer and deserializer.
+
+### Adding a new benchmark
+
+To add a new benchmark, first add the file you'd like to measure performance against to the `files` directory. Then, add a new benchmark function to `suite/main.rs`, like this:
+```rust
+pub fn my_benchmark(c: &mut Criterion) {
+    bench(
+        &mut c.benchmark_group("My Benchmark")
+        include_bytes!("../files/bench-file.rbxl"),
+    )
+}
+```
+and also make sure to add your benchmark function to the `criterion_group!` macro invocation.
+
+Benchmark groups provide a number of configuration options which are useful under different circumstances. See the [Criterion.rs benchmark configuration documentation](https://bheisler.github.io/criterion.rs/book/user_guide/advanced_configuration.html) for details.
+
+### Running the benchmarks
+
+To run all benchmarks, run the following command somewhere in the `rbx_binary` crate directory:
+```bash
+cargo bench
+```
+
+To run a specific benchmark, run the following command somewhere in the `rbx_binary` crate directory, subsituting `My Benchmark` with the name of the benchmark group:
+```bash
+cargo bench "My Benchmark"
+```
+
+To measure only serialization or deserialization, add `/Serialize` or `/Deserialize` to the end of the benchmark name, like this:
+```bash
+cargo bench "My Benchmark/Serialize"
+```
+
+Once the benchmark is complete, an HTML report will be generated at `rbx-dom/target/criterion/reports/index.html` that contains detailed statistics collected during the run. This file can be opened in a web browser.
+
+For more information, see the [Criterion.rs documentation](https://bheisler.github.io/criterion.rs/book/).