Update backend-storage.md (#2915)

docs/en/setup/backend/backend-storage.md

@@ -76,6 +76,8 @@ storage:
     concurrentRequests: ${SW_STORAGE_ES_CONCURRENT_REQUESTS:2} # the number of concurrent requests
 ```
 
+Read the [ElasticSearch storage FAQ](../../FAQ/ES-Server-FAQ.md) if you are new to ElasticSearch. And recommend read more about these configuration from ElasticSearch official document. This effects the performance of ElasticSearch very much.
+
 ### ElasticSearch 6 with Jaeger trace extension
 This implementation shares most of `elasticsearch`, just extend to support zipkin span storage.
 It has all same configs.