modify README.md to add CoKE arxiv link and citation

PaddleKG/CoKE/README.md

 # CoKE: Contextualized Knowledge Graph Embedding
 ## Introduction
 
-This is the [PaddlePaddle](https://www.paddlepaddle.org.cn/) implementation of the CoKE model for Knowledge Graph Embedding(KGE).
+This is the [PaddlePaddle](https://www.paddlepaddle.org.cn/) implementation of the [CoKE](https://arxiv.org/abs/1911.02168) model for Knowledge Graph Embedding(KGE).
 
 CoKE is a novel KGE paradigm that learns dynamic, flexible, and fully contextualized entity and relation representations for a given Knowledge Graph(KG).
 It takes a sequence of entities and relations as input, and uses [Transformer](https://arxiv.org/abs/1706.03762) to obtain contextualized representations for its components.
@@ -159,6 +159,18 @@ These are also given in the `configs/${TASK}_job_config.sh` files.
 |pathqueryFB | L=6, H=256, A=4 | 3e-4 | 1 | 10 | 2048 | 0.1 |
 |pathqueryWN | L=6, H=256, A=4 | 3e-4 | 1 | 5 | 2048 | 0.1 |
 
+## Citation
+If you use any source code included in this project in your work, please cite the following paper:
+
+```
+@article{wang2019:coke,
+  title={CoKE: Contextualized Knowledge Graph Embedding},
+  author={Wang, Quan and Huang, Pingping and Wang, Haifeng and Dai, Songtai and Jiang, Wenbin and Liu, Jing and Lyu, Yajuan and Wu, Hua},
+  journal={arXiv:1911.02168},
+  year={2019}
+}
+```
+
 
 ## Copyright and License
 Copyright 2019 Baidu.com, Inc. All Rights Reserved Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.