Add changelog entry

Signed-off-by: Andy Asp <[email protected]>

CHANGELOG.md

@@ -15,6 +15,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 - [#34](https://github.com/thanos-io/objstore/pull/34) Fix ignored options when creating shared credential Azure client.
 - [#62](https://github.com/thanos-io/objstore/pull/62) S3: Fix ignored context cancellation in `Iter` method.
 - [#77](https://github.com/thanos-io/objstore/pull/77) Fix buckets wrapped with metrics from being unable to determine object sizes in `Upload`.
+- [#78](https://github.com/thanos-io/objstore/pull/78) S3: Fix possible concurrent modification of the PutUserMetadata map.
 
 ### Added
 - [#15](https://github.com/thanos-io/objstore/pull/15) Add Oracle Cloud Infrastructure Object Storage Bucket support.