docs: Add bibtex ref

docs/user/cite_idrlnet.md

@@ -1,2 +1,11 @@
 # Cite IDRLnet
-The paper is to appear on Arxiv.
+```
+@misc{peng2021idrlnet,
+      title={IDRLnet: A Physics-Informed Neural Network Library}, 
+      author={Wei Peng and Jun Zhang and Weien Zhou and Xiaoyu Zhao and Wen Yao and Xiaoqian Chen},
+      year={2021},
+      eprint={2107.04320},
+      archivePrefix={arXiv},
+      primaryClass={cs.LG}
+}
+```