Update changelog

Author: cpsievert

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to Shiny for Python will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [UNRELEASED]
+
+### New features
+
+* Added a new `input_submit_textarea()` input element, which is similar to `input_text_area()`, but includes a submit button to only submit the text changes to the server on click. This is especially useful when the input text change triggers a long-running operation and/or the user wants to type longer-form input and review it before submitting it. (#2099)
+
 ## [1.5.0] - 2025-09-11
 
 ### New features