docs(sessions): add memcache support to changelog

CHANGELOG.md

@@ -5,6 +5,9 @@
 **InfluxDB 0.8.x** The data source for the old version of influxdb (0.8.x) is no longer included in default builds. Can easily be installed via improved plugin system, closes #3523
 **KairosDB** The data source is no longer included in default builds. Can easily be installed via improved plugin system, closes #3524
 
+### Enhancements ###
+* **Sessions**: Support for memcached as session storage, closes [#3458](https://github.com/grafana/grafana/pull/3458)
+
 # 2.6.1 (unrelased, 2.6.x branch)
 
 ### New Features