Update benchmark.md

appendix/benchmark.md

@@ -1,7 +1,12 @@
 # Benchmark results
 
+## Usage
 
-## dataset
+```
+th tools/benchmark.lua -dir path/to/dataset_dir -method scale -color y -model1_dir path/to/model_dir
+```
+
+## Dataset
 
     photo_set: 300 various photos.
     art_set  : 90 artworks (PNG only).