Update documentation for new Ractor benchmarks

Author: eightbitraptor

README.md

@@ -101,6 +101,43 @@ It's also convenient for profiling, debugging, etc, especially since all benchma
 ruby benchmarks/some_benchmark.rb
 ```
 
+## Ractor Benchmarks
+
+ruby-bench supports Ractor-specific benchmarking with dedicated categories and benchmark directories.
+
+### Ractor Categories
+
+There are two Ractor-related categories:
+
+* **`--category ractor`** - Runs both regular benchmarks marked with `ractor:
+  true` in `benchmarks.yml` AND all benchmarks from the `benchmarks-ractor`
+  directory. The `harness-ractor` harness is used for both types of benchmark.
+
+* **`--category ractor-only`** - Runs ONLY benchmarks from the
+  `benchmarks-ractor` directory, ignoring regular benchmarks even if they are
+  marked with `ractor: true`. This category also automatically uses the
+  `harness-ractor` harness.
+
+### Directory Structure
+
+The `benchmarks-ractor/` directory sits at the same level as the main
+`benchmarks` directory, and contains Ractor-specific benchmark
+implementations that are designed to test Ractor functionality. They are not
+intended to be used with any harness except `harness-ractor`.
+
+### Usage Examples
+
+```bash
+# Run all Ractor-capable benchmarks (both regular and Ractor-specific)
+./run_benchmarks.rb --category ractor
+
+# Run only dedicated Ractor benchmarks from benchmarks-ractor directory
+./run_benchmarks.rb --category ractor-only
+```
+
+Note: The `harness-ractor` harness is automatically selected when using these
+categories, so there's no need to specify `--harness` manually.
+
 ## Ruby options
 
 By default, ruby-bench benchmarks the Ruby used for `run_benchmarks.rb`.