docs: add benchmark section to README

Adds a benchmark section to the README comparing the Go and Rust implementations of auto-commit-msg, using hyperfine for benchmarking. Also updates the .gitignore to include benchmark results and adds hyperfine to the devenv.nix configuration.
---
{"auto-commit-msg":{"language":"rust","version":"0.4.0-dev","model":"gemini-2.5-flash-lite","response_time":1.31,"execution_time":1.33}}

Author: sestrella

.gitignore

@@ -11,6 +11,7 @@ devenv.local.nix
 # Project specific
 .env
 auto-commit-msg
+benchmark.md
 result
 templates/default/devenv.lock
 

README.md

@@ -109,6 +109,16 @@ commit` without a commit message generates a commit message. If a commit message
 is given, `auto-commit-msg` does not generate a commit message and instead uses
 the one provided by the user.
 
+## Benchmark
+
+The following table compares the Go and Rust implementations; it was built using
+[hyperfine](https://github.com/sharkdp/hyperfine).
+
+| Command | Mean [s] | Min [s] | Max [s] | Relative |
+|:---|---:|---:|---:|---:|
+| `./auto-commit-msg` | 1.101 ± 0.205 | 0.776 | 1.309 | 1.00 |
+| `./rust/target/release/acm` | 1.244 ± 0.106 | 1.127 | 1.416 | 1.13 ± 0.23 |
+
 ## License
 
 [MIT](LICENSE)

devenv.nix

@@ -14,6 +14,7 @@
     pkgs.gitleaks
     pkgs.gomod2nix
     pkgs.goreleaser
+    pkgs.hyperfine
     pkgs.secretspec
   ];