add ref of non-local

PaddleCV/video/README.md

@@ -127,7 +127,7 @@ infer.py
 - [StNet:Local and Global Spatial-Temporal Modeling for Human Action Recognition](https://arxiv.org/abs/1811.01549), Dongliang He, Zhichao Zhou, Chuang Gan, Fu Li, Xiao Liu, Yandong Li, Limin Wang, Shilei Wen
 - [Temporal Segment Networks: Towards Good Practices for Deep Action Recognition](https://arxiv.org/abs/1608.00859), Limin Wang, Yuanjun Xiong, Zhe Wang, Yu Qiao, Dahua Lin, Xiaoou Tang, Luc Van Gool
 - [Temporal Shift Module for Efficient Video Understanding](https://arxiv.org/abs/1811.08383), Ji Lin, Chuang Gan, Song Han
-
+- [Non-local Neural Networks](https://arxiv.org/abs/1711.07971v1), Xiaolong Wang, Ross Girshick, Abhinav Gupta, Kaiming He
 ## 版本更新
 
 - 3/2019: 新增模型库，发布Attention Cluster，Attention LSTM，NeXtVLAD，StNet，TSN五个视频分类模型。