add changelog

Signed-off-by: Ben Ye <yb532204897@gmail.com>

Author: yeya24

CHANGELOG.md

@@ -23,6 +23,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#3356](https://github.com/thanos-io/thanos/pull/3356) Query Frontend: Add a flag to disable step alignment middleware for query range.
 - [#3378](https://github.com/thanos-io/thanos/pull/3378) Ruler: added the ability to send queries via the HTTP method POST. Helps when alerting/recording rules are extra long because it encodes the actual parameters inside of the body instead of the URI. Thanos Ruler now uses POST by default unless `--query.http-method` is set `GET`.
 - [#2979](https://github.com/thanos-io/thanos/pull/2979) Replicator: Add the ability to replicate blocks within a time frame by passing --min-time and --max-time
+- [#3398](https://github.com/thanos-io/thanos/pull/3398) Query Frontend: Add default config for query frontend memcached config.
 
 ### Fixed
 - [#3257](https://github.com/thanos-io/thanos/pull/3257) Ruler: Prevent Ruler from crashing when using default DNS to lookup hosts that results in "No such hosts" errors.