Update changelog

Author: cpsievert

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [UNRELEASED]
 
+## New features
+
+* Both `ui.Chat()` and `ui.MarkdownStream()` now support arbirary Shiny UI elements inside of messages. This allows for gathering input from the user (e.g., `ui.input_select()`), displaying of rich output (e.g., `render.DataGrid()`), and more. (#1868)
+
 ### Changes
 
 * Express mode's `app_opts()` requires all arguments to be keyword-only. If you are using positional arguments, you will need to update your code. (#1895)