Add paper lists (#61)

* add FL paper list

materials/paper_list/FL-Attacker/README.md

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+The paper list goes here.

materials/paper_list/FL-Database/README.md

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+The paper list goes here.

materials/paper_list/FL-HPO/README.md

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+The paper list goes here.

materials/paper_list/FL-Incentive/README.md

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+The paper list goes here.

materials/paper_list/FL-Recommendation/README.md

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-## The paper list of Federated Learning in Recommendation will go here.
+## Federated Learning for Recommendation
+
+### 2021
+| Title | Venue | Link |  
+| --- | --- | --- | 
+| Efficient-FedRec: Efficient Federated Learning Framework for Privacy-Preserving News Recommendation | EMNLP |  [pdf](https://aclanthology.org/2021.emnlp-main.223), [code](https://github.com/yjw1029/Efficient-FedRec) |
+
+### 2020
+| Title | Venue | Link |  
+| --- | --- | --- | 
+| Secure Federated Matrix Factorization | IEEE Intelligent Systems | [pdf](https://ieeexplore.ieee.org/abstract/document/9162459), [code](https://github.com/Di-Chai/FedMF) | 
+| Privacy-Preserving News Recommendation Model Learning | EMNLP | [pdf](https://aclanthology.org/2020.findings-emnlp.128/), [code](https://github.com/taoqi98/FedNewsRec) |
+
+### 2019
+| Title | Venue | Link |  
+| --- | --- | --- | 
+| Federated Collaborative Filtering for Privacy-Preserving Personalized Recommendation System | arXiv | [pdf](https://arxiv.org/abs/1901.09888) |
+