Mention timeline_agg in changelog

Changelog.md

@@ -30,6 +30,8 @@ This changelog should be updated as part of a PR if the work is worth noting (mo
 
   Users can use the new `toolkit_experimental.approx_percentile_array(percentiles)` to generate an array of percentile results instead of having to call and rebuild the aggregate multiple times.
 
+- [#636](https://github.com/timescale/timescaledb-toolkit/pull/636): New `timeline_agg` aggregate, which is similar to `state_agg` but tracks the entire state timeline instead of just the duration in each state.
+
 #### Bug fixes
 - [#644](https://github.com/timescale/timescaledb-toolkit/pull/644): Fix bug in Candlestick aggregate and reenable partial aggregation.