docs(elasticsearch-plugin): Add note on minimum version of Elasticsearch

packages/elasticsearch-plugin/README.md

@@ -1,6 +1,8 @@
 # Vendure Elasticsearch Plugin
 
-The `ElasticsearchPlugin` uses Elasticsearch to power the the Vendure product search.
+The `ElasticsearchPlugin` uses Elasticsearch to power the the Vendure product search. 
+
+**Requires Elasticsearch v7.0 or higher.** 
 
 `npm install @vendure/elasticsearch-plugin`
 

packages/elasticsearch-plugin/src/plugin.ts

@@ -33,6 +33,8 @@ import { ElasticsearchOptions, mergeWithDefaults } from './options';
  *
  * ## Installation
  *
+ * **Requires Elasticsearch v7.0 or higher.**
+ *
  * `yarn add \@vendure/elasticsearch-plugin`
  *
  * or