chore: update README.md

README.md

@@ -204,6 +204,7 @@ Available targets:
 | Name | Description |
 |------|-------------|
 | <a name="output_arn"></a> [arn](#output\_arn) | Amazon Resource Name (ARN) of the cluster |
+| <a name="output_cluster_members"></a> [cluster\_members](#output\_cluster\_members) | List of DocumentDB Instances that are a part of this cluster |
 | <a name="output_cluster_name"></a> [cluster\_name](#output\_cluster\_name) | Cluster Identifier |
 | <a name="output_endpoint"></a> [endpoint](#output\_endpoint) | Endpoint of the DocumentDB cluster |
 | <a name="output_master_host"></a> [master\_host](#output\_master\_host) | DB master hostname |