changelog: add notes about 5.2.0-beta2

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CHANGELOG.md

@@ -4,6 +4,8 @@
 
 # 5.2.0 (unreleased)
 
+# 5.2.0-beta2 (2018-06-20)
+
 ### New Features
 
 * **Dashboard**: Import dashboard to folder [#10796](https://github.com/grafana/grafana/issues/10796)