Make ZooKeeper Version Compatibility clear in document (#1916)

* ZooKeeper Version Compatibility

* Update backend-cluster.md

Reorganize words. cc @peng-yongsheng

* Update application.yml

* Update application.yml

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

@@ -12,7 +12,13 @@ by using k8s native APIs to manage cluster.
 
 ## Zookeeper coordinator
 Zookeeper is a very common and wide used cluster coordinator. Set the **cluster** module's implementor
-to **zookeeper** in the yml to active.
+to **zookeeper** in the yml to active. 
+
+Required Zookeeper version, 3.5+
+
+Zookeeper version is required above 3.5 by Curator 4.0 lib, but actually Curator 4.0 is compatible with ZooKeeper 3.4.x.
+To make ZooKeeper 3.4.x works, please replace the ZooKeeper 3.5+ library jar in `oap-libs` folder with ZooKeeper 3.4.x library.
+
 ```yaml
 cluster:
   zookeeper:

oap-server/server-starter/src/main/assembly/application.yml

@@ -16,6 +16,8 @@
 
 cluster:
   standalone:
+  # Please check your ZooKeeper is 3.5+, However, it is also compatible with ZooKeeper 3.4.x. Replace the ZooKeeper 3.5+
+  # library in the oap-libs folder with your ZooKeeper 3.4.x libraru.
 #  zookeeper:
 #    hostPort: localhost:2181
 #    # Retry Policy

oap-server/server-starter/src/main/resources/application.yml

@@ -16,6 +16,8 @@
 
 cluster:
   standalone:
+   # Please check your ZooKeeper is 3.5+, However, it is also compatible with ZooKeeper 3.4.x. Replace the ZooKeeper 3.5+
+   # library the oap-libs folder with your ZooKeeper 3.4.x library.
 #  zookeeper:
 #    hostPort: localhost:2181
 #    # Retry Policy