Merge pull request #3286 from wayilau/doc_fix1

Add cluster etcd config to cluster.md.

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

@@ -10,6 +10,7 @@ with each other.
 by using k8s native APIs to manage cluster.
 - [Consul](#consul). Use Consul as backend cluster management implementor, to coordinate backend instances.
 - [Nacos](#nacos). Use Nacos to coordinate backend instances.
+- [Etcd](#etcd). Use Etcd to coordinate backend instances.
 
 ## Zookeeper coordinator
 Zookeeper is a very common and wide used cluster coordinator. Set the **cluster** module's implementor
@@ -90,4 +91,16 @@ cluster:
     serviceName: ${SW_SERVICE_NAME:"SkyWalking_OAP_Cluster"}
     # Nacos cluster nodes, example: 10.0.0.1:8848,10.0.0.2:8848,10.0.0.3:8848
     hostPort: ${SW_CLUSTER_NACOS_HOST_PORT:localhost:8848}
+```
+
+## Etcd
+Set the **cluster** module's implementor to **etcd** in
+the yml to active.
+
+```yaml
+cluster:
+  etcd:
+    serviceName: ${SW_SERVICE_NAME:"SkyWalking_OAP_Cluster"}
+    #etcd cluster nodes, example: 10.0.0.1:2379,10.0.0.2:2379,10.0.0.3:2379
+    hostPort: ${SW_CLUSTER_ETCD_HOST_PORT:localhost:2379}
 ```
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