docs(playlists): add persisted playlists to changelog

CHANGELOG.md

 # 3.0.0 (unrelased master branch)
 
+### New Features ###
+* **Playlists**: Playlists can now be persisted and started from urls, closes [#3655](https://github.com/grafana/grafana/pull/3655)
+
 ### Breaking changes
 **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