Update README.md

Author: layumi

tutorial/README.md

@@ -305,6 +305,9 @@ except RuntimeError:
  print('If you want to see the visualization of the ranking result, graphical user interface is needed.')
 ```
 
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+## Reference
+
 [1] Deng, Jia, Wei Dong, Richard Socher, Li-Jia Li, Kai Li, and Li Fei-Fei. "Imagenet: A large-scale hierarchical image database." In Computer Vision and Pattern Recognition, 2009. CVPR 2009. IEEE Conference on, pp. 248-255. Ieee, 2009.