docs: Add changelog entry

Author: jmartinesp

bindings/matrix-sdk-ffi/CHANGELOG.md

@@ -34,6 +34,11 @@ All notable changes to this project will be documented in this file.
   `top_level_joined_spaces` and `subscribe_to_joined_spaces` to `space_service.subscribe_to_top_level_joined_spaces`
   ([#5972](https://github.com/matrix-org/matrix-rust-sdk/pull/5972))
 
+### 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