chore: add CITATION.md (leanprover-community#34708)

This PR adds a CITATION.md file with bibtex for citing the mathlib CPP 2020 paper.

🤖 Prepared with Claude Code

Author: kim-em

CITATION.md

@@ -0,0 +1,24 @@
+# Citing mathlib
+
+If you use mathlib in your work, please cite:
+
+> The mathlib Community. *The Lean Mathematical Library.* CPP 2020.
+> https://doi.org/10.1145/3372885.3373824
+
+## BibTeX
+
+```bibtex
+@inproceedings{mathlib2020,
+  author    = {{The mathlib Community}},
+  title     = {The {L}ean {M}athematical {L}ibrary},
+  booktitle = {Proceedings of the 9th {ACM} {SIGPLAN} International Conference
+               on Certified Programs and Proofs},
+  series    = {CPP 2020},
+  publisher = {ACM},
+  address   = {New Orleans, LA, USA},
+  year      = {2020},
+  month     = jan,
+  doi       = {10.1145/3372885.3373824},
+  url       = {https://doi.org/10.1145/3372885.3373824}
+}
+```