MAINT: update changelog for layer chart marks
## Version 3.1.0 (unreleased)
+### Enhancements
+
+- ``alt.LayerChart`` now supports ``mark_*()`` methods. If a layer specifies a
+ mark at the top level, all child charts will inherit it (unless they override
+ it explicitly).
+
## Version 3.0.1
Fix version info bug for HTML output and Colab & Kaggle renderers.
