Docs: Add missing changelog entry for stream sharding (grafana#7622)

**What this PR does / why we need it**:
Add missing changelog entry for the new stream sharding feature.
This will be better detailed under the release notes doc.

CHANGELOG.md

@@ -60,6 +60,7 @@ Check the history of the branch FIXME.
 * [7264](https://github.com/grafana/loki/pull/7264) **bboreham**: Chunks: decode varints directly from byte buffer, for speed.
 * [7263](https://github.com/grafana/loki/pull/7263) **bboreham**: Dependencies: klauspost/compress package to v1.15.11; improves performance.
 * [7270](https://github.com/grafana/loki/pull/7270) **wilfriedroset**: Add support for `username` to redis cache configuration.
+* [6952](https://github.com/grafana/loki/pull/6952) **DylanGuedes**: Experimental: Introduce a new feature named stream sharding.
 
 ##### Fixes
 * [7453](https://github.com/grafana/loki/pull/7453) **periklis**: Add single compactor http client for delete and gennumber clients