docs: add two high-cited survey on ppml (secretflow#1013)

docs/awesome-pets/papers/applications/ppml/ppml_crypto.md

@@ -89,6 +89,15 @@ An overview of existing works is illustrated in the table below.
     *HAN Wei-Li SONG Lu-shan RUAN Wen-qiang LIN Guo-peng WANG Zhe-xuan*
     Chinese Journal of Computers, [eprint in Chinese](http://cjc.ict.ac.cn/online/onlinepaper/hwl-202375100742.pdf)
 
+- When Machine Learning Meets Privacy: A Survey and Outlook.
+    *Bo Liu, Ming Ding, Sina Shaham, Wenny Rahayu, Farhad Farokhi, and Zihuai Lin* 
+    ACM Computing Surveys (CSUR), [eprint](https://arxiv.org/pdf/2011.11819.pdf)
+
+
+- Privacy-preserving machine learning: Methods, challenges and directions.
+    *Xu R, Baracaldo N, Joshi J* 
+    arXiv preprint arXiv, [eprint](https://arxiv.org/pdf/2108.04417.pdf)
+
 ## Two-party Computation (2PC)
 
 ### <a id='2pc-infer'>Secure Inference</a>