doc: Add changelog entry

Author: jmartinesp

bindings/matrix-sdk-ffi/CHANGELOG.md

@@ -13,6 +13,11 @@ All notable changes to this project will be documented in this file.
   documentation of `matrix_sdk::latest_event` to learn about the new API.
   [#5624](https://github.com/matrix-org/matrix-rust-sdk/pull/5624/)
 
+### Breaking changes
+
+- The existing `TimelineEventType` was renamed to `TimelineEventContent`, because it contained the actual contents 
+  of the event. Then, we created a new `TimelineEventType` enum that actually contains *just* the event type.
+
 ## [0.16.0] - 2025-12-04
 
 ### Breaking changes