Add changelog entry

Signed-off-by: Filip Petkovski <filip.petkovsky@gmail.com>

CHANGELOG.md

@@ -21,7 +21,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 ### Added
 - [#7105](https://github.com/thanos-io/thanos/pull/7105) Rule: add flag `--query.enable-x-functions` to allow usage of extended promql functions (xrate, xincrease, xdelta) in loaded rules
 - [#6867](https://github.com/thanos-io/thanos/pull/6867) Query UI: Tenant input box added to the Query UI, in order to be able to specify which tenant the query should use.
-
+- [#7175](https://github.com/thanos-io/thanos/pull/7175): Query: Add `--query.mode=distributed` which enables the new distributed mode of the Thanos query engine.
 ### Changed
 
 - [#7123](https://github.com/thanos-io/thanos/pull/7123) Rule: Change default Alertmanager API version to v2.