Add email to readme

Author: xumingkuan

README.md

@@ -166,6 +166,10 @@ Explanation for some of the parameters:
 
 See [code structure](CODE_STRUCTURE.md) for more information about the organization of the Quartz code base.
 
+## Issues
+
+Please file an issue or contact [email protected] if you encounter any problems.
+
 ## Contributing
 
 Please let us know if you encounter any bugs or have any suggestions by [submitting an issue](https://github.com/quantum-compiler/quartz/issues).
@@ -174,8 +178,6 @@ We welcome all contributions to Quartz from bug fixes to new features and extens
 
 Please follow [developer guidance](doc/dev_setup.md).
 
-Please subscribe to the Quartz users mailing list (TODO)
-
 ## Citations
 
 * Mingkuan Xu, Zikun Li, Oded Padon, Sina Lin, Jessica Pointing, Auguste Hirth, Henry Ma, Jens Palsberg, Alex Aiken, Umut A. Acar, and Zhihao Jia. [Quartz: Superoptimization of Quantum Circuits](https://arxiv.org/abs/2204.09033). In Proceedings of the Conference on Programming Language Design and Implementation (PLDI), June 2022.