Add Wang et al to citations.

This adds this paper: https://www.aaai.org/Papers/AAAI/2020GB/AAAI-WangS.1218.pdf

Author: drvinceknight

citations.md

@@ -3,6 +3,7 @@
 ### 2020
 
 * Glynatsi, Nikoleta E., and Vincent A. Knight. "A meta analysis of tournaments and an evaluation of performance in the Iterated Prisoner's Dilemma" arXiv preprint arXiv:2001.05911 (2019). [ArXiv](https://arxiv.org/abs/2001.05911)
+* Wang, Shiheng, and Fangzhen Lin. "Nice Invincible Strategy for the Average-Payoff IPD." https://www.aaai.org/Papers/AAAI/2020GB/AAAI-WangS.1218.pdf
 
 ### 2019