Add comments to describe that benchmark result is outdated.

appendix/benchmark.md

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 # Benchmark results
 
+Warning: This benchmark results is outdated. I will update soon.
+
 ## Usage
 
 ```

appendix/vgg_7_vs_upconv_7_art.md

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 # models/vgg_7/art vs models/upconv_7/art
 
+Warning: This benchmark results is outdated. I will update soon.
+
 ## dataset
 
 noise free 84 anime style arts. PNG format only.