Add arXiv citation at end

README.md

@@ -115,6 +115,17 @@ Headline numbers in Table 1 of the paper (the OPERA-MAPGRPO row)
 require MAPGRPO training of the three agents; the training code is not
 part of this release.
 
+## Citation
+
+```bibtex
+@article{Liu2026OPERA,
+  author  = {Liu, Yu and Liu, Yanbing and Yuan, Fangfang and Cao, Cong and Sun, Youbang and Peng, Kun and Chen, WeiZhuo and Li, Jianjun and Ma, Zhiyuan},
+  title   = {{OPERA}: A Reinforcement Learning--Enhanced Orchestrated Planner-Executor Architecture for Reasoning-Oriented Multi-Hop Retrieval},
+  journal = {arXiv preprint arXiv:2508.16438},
+  year    = {2026}
+}
+```
+
 ## License
 
 The retrieval index and metadata are released for research use only.