docs(changelog): add info about cli tool

CHANGELOG.md

@@ -6,6 +6,7 @@
 * **InfluxDB**: Support for policy selection in query editor, closes [#2018](https://github.com/grafana/grafana/issues/2018)
 * **Snapshots UI**: Dashboard snapshots list can be managed through UI, closes[#1984](https://github.com/grafana/grafana/issues/1984)
 * **Prometheus**: Prometheus annotation support, closes[#2883](https://github.com/grafana/grafana/pull/2883)
+* **Cli**: New cli tool for downloading and updating plugins
 
 ### Breaking changes
 * **Plugin API**: Both datasource and panel plugin api (and plugin.json schema) have been updated, requiring an update to plugins. See [plugin api](https://github.com/grafana/grafana/blob/master/public/app/plugins/plugin_api.md) for more info.