Updated README.md with recent releases

Author: vvanholl

README.md

@@ -62,6 +62,8 @@ It collects all relevant metrics and makes them available to Prometheus via the
 
 | Elasticsearch  | Plugin         | Release date |
 | -------------- | -------------- | ------------ |
+| 5.6.16         | 5.6.16.0       | Apr 04, 2019 |
+| 5.6.15         | 5.6.15.0       | Feb 21, 2019 |
 | 5.6.14         | 5.6.14.0       | Dec 14, 2018 |
 | 5.6.13         | 5.6.13.1       | Nov 12, 2018 |
 | 5.6.13         | 5.6.13.0       | Nov 07, 2018 |