Add changelog

Signed-off-by: Saswata Mukherjee <[email protected]>

CHANGELOG.md

@@ -21,6 +21,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 ### Changed
 
 - [#5255](https://github.com/thanos-io/thanos/pull/5296) Query: Use k-way merging for the proxying logic. The proxying sub-system now uses much less resources (~25-80% less CPU usage, ~30-50% less RAM usage according to our benchmarks). Reduces query duration by a few percent on queries with lots of series.
+- [#5685](https://github.com/thanos-io/thanos/pull/5685) Receive: Make active/head series limiting configuration per tenant by adding it to new limiting config.
 
 ### Removed