highlight elasticsearch supported version range

Close #2581

Author: wey-gu

docs-2.0-en/4.deployment-and-installation/6.deploy-text-based-index/2.deploy-es.md

@@ -13,6 +13,7 @@ To deploy an Elasticsearch cluster, see [Kubernetes Elasticsearch deployment](ht
 !!! note
 
     To support external network access to Elasticsearch, set `network.host` to `0.0.0.0` in `config/elasticsearch.yml`.
+    Only elastvsearch in version 7.x is supported.
 
 You can configure the Elasticsearch to meet your business needs. To customize the Elasticsearch, see [Elasticsearch Document](https://www.elastic.co/guide/en/elasticsearch/reference/current/settings.html).