Docs: Updated changelog with v6.1.2 release issues

CHANGELOG.md

 # 6.2.0 (unreleased)
 
+# 6.1.2 (2019-04-08)
+
+ ### Bug Fixes
+  * **Graph**: Fixed series legend color for hidden series. [#16438](https://github.com/grafana/grafana/pull/16438), [@Ijin08](https://github.com/Ijin08)
+  * **Graph**: Fixed tooltip highlight on white theme. [#16429](https://github.com/grafana/grafana/pull/16429), [@torkelo](https://github.com/torkelo)
+  * **Styles**: Fixed menu hover highlight border. [#16431](https://github.com/grafana/grafana/pull/16431), [@torkelo](https://github.com/torkelo)
+  * **Units**: Correctly use the override decimals. [#16413](https://github.com/grafana/grafana/pull/16413), [@torkelo](https://github.com/torkelo)
+
 # 6.1.1 (2019-04-05)
 
  ### Bug Fixes