README.md in wrapbox-0.3.0 vs README.md in wrapbox-0.4.0

@@ -83,9 +83,20 @@
 | region                           | region of ECS cluster                                                                                       |
 | container_definition             | see. http://docs.aws.amazon.com/sdkforruby/api/Aws/ECS/Client.html#register_task_definition-instance_method |
 | additional_container_definitions | Container definitions for sub containers                                                                    |
 | task_role_arn                    | see. http://docs.aws.amazon.com/AmazonECS/latest/developerguide/task-iam-roles.html                         |
 
+`WRAPBOX_CMD_INDEX` environment variable is available in `run_cmd` and you can distinguish logs from each command like below:
+
+```
+log_configuration:
+  log_driver: syslog
+  options:
+    syslog-address: "tcp://192.168.0.42:123"
+    env: WRAPBOX_CMD_INDEX
+    tag: wrapbox-{{ printf "%03s" (.ExtraAttributes nil).WRAPBOX_CMD_INDEX }}
+```
+
 ### for docker
 | name                 | desc                                                        |
 | -------------------- | ----------------------------------------------------------- |
 | container_definition | only use `image`, `cpu`, `memory`, and `memory_reservation` |
 | rm                   | If true, add `--rm` to cmd options                          |