Update changelog

Signed-off-by: Hitanshu Mehta <hitanshu99amehta@gmail.com>

CHANGELOG.md

@@ -14,6 +14,7 @@ We use _breaking :warning:_ to mark changes that are not backward compatible (re
 
 ### Added
 
+- [#3740](https://github.com/thanos-io/thanos/pull/3740) Query: Added `--query.default-set` flag to set default step.
 - [#3700](https://github.com/thanos-io/thanos/pull/3700) ui: make old bucket viewer UI work with vanilla Prometheus blocks
 - [#2641](https://github.com/thanos-io/thanos/issues/2641) Query Frontend: Added `--query-range.request-downsampled` flag enabling additional queries for downsampled data in case of empty or incomplete response to range request.
 - [#3792](https://github.com/thanos-io/thanos/pull/3792) Receiver: Added `--tsdb.allow-overlapping-blocks` flag to allow overlapping tsdb blocks and enable vertical compaction