Add instructions for running benchmarks

Author: john-shaffer

README.md

@@ -1 +1,16 @@
 # WordPress Cache Benchmarks
+
+## Running Benchmarks
+
+Running the benchmarks requires installing
+[Nix](https://docs.determinate.systems/determinate-nix/#getting-started).
+You can enter a development shell via `nix develop`,
+or automatically using [direnv](https://direnv.net/).
+
+After starting a development shell,
+you can run the benchmarks via `just bench`.
+This runs a series of 500 random requests using each
+plugin and each backend that that plugin supports.
+
+You can run specific benchmarks via `just bench -c <name>`.
+See `finefile --help` for a full list of options.