highlight citation in README

README.md

@@ -10,13 +10,21 @@ algorithms in a flexible fashion suitable for research and education.
 
 LensKit for Python (LKPY) is the successor to the Java-based LensKit project.
 
-If you use LensKit for Python in published research, please cite:
-
-> Michael D. Ekstrand. 2020.
-> LensKit for Python: Next-Generation Software for Recommender Systems Experiments.
-> In <cite>Proceedings of the 29th ACM International Conference on Information and Knowledge Management</cite> (CIKM '20).
-> DOI:[10.1145/3340531.3412778](https://dx.doi.org/10.1145/3340531.3412778).
-> arXiv:[1809.03125](https://arxiv.org/abs/1809.03125) [cs.IR].
+> [!IMPORTANT]
+> If you use LensKit for Python in published research, please cite:
+>
+> > Michael D. Ekstrand. 2020.
+> > LensKit for Python: Next-Generation Software for Recommender Systems Experiments.
+> > In <cite>Proceedings of the 29th ACM International Conference on Information and Knowledge Management</cite> (CIKM '20).
+> > DOI:[10.1145/3340531.3412778](https://dx.doi.org/10.1145/3340531.3412778).
+> > arXiv:[1809.03125](https://arxiv.org/abs/1809.03125) [cs.IR].
+
+> [!WARNING]
+> This is the `main` branch of LensKit, following new development in preparation
+> for the 2024 release.  It will be changing frequently and incompatibly. You
+> probably want to use a [stable release][release]
+
+[release]: https://lkpy.lenskit.org/en/stable/
 
 ## Installing