Update to publications list with AAMAS 2021 SPOTTER paper. (#137)

Co-authored-by: Vasanth Sarathy <vsarathy@system76-pc.localdomain>

README.md

@@ -29,6 +29,7 @@ Please use this bibtex if you want to cite this repository in your publications:
 ```
 
 List of publications & submissions using MiniGrid or BabyAI (please open a pull request to add missing entries):
+- [SPOTTER: Extending Symbolic Planning Operators through Targeted Reinforcement Learning](http://www.ifaamas.org/Proceedings/aamas2021/pdfs/p1118.pdf) (Tufts University, SIFT, AAMAS 2021)
 - [Grid-to-Graph: Flexible Spatial Relational Inductive Biases for Reinforcement Learning](https://arxiv.org/abs/2102.04220) (UCL, AAMAS 2021)
 - [Rank the Episodes: A Simple Approach for Exploration in Procedurally-Generated Environments](https://openreview.net/forum?id=MtEE0CktZht) (Texas A&M University, Kuai Inc., ICLR 2021)
 - [Adversarially Guided Actor-Critic](https://openreview.net/forum?id=_mQp5cr_iNy) (INRIA, Google Brain, ICLR 2021)