Add cluster etcd config to cluster.md.

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

@@ -90,4 +90,16 @@ 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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