Adding Elasticsearch in supported databases

Author: jsargiot

README.md

@@ -14,6 +14,7 @@ Supported Databases:
 - InfluxDB
 - OpenTSDB
 - Prometheus
+- Elasticsearch
 
 Included location data:
 
@@ -41,6 +42,12 @@ The Group By clause should be the country code and an alias is needed too. The a
 
 ![Influx Query for Countries](https://raw.githubusercontent.com/grafana/worldmap-panel/master/src/images/influx-query.png)
 
+#### Elasticseach Query for Countries
+
+Use a Group By clause on the field containing the country code and a Then by clause with Date Histogram by `@timestamp` (or corresponding date field).
+
+![Elasticsearch Query for Countries](https://raw.githubusercontent.com/grafana/worldmap-panel/master/src/images/elasticsearch-query.png)
+
 #### Map Data Options for Time Series Data
 
 Under the Worldmap tab, choose either the `countries` or `states` option.