Update docker plugin README (#3404)

plugins/inputs/docker/README.md

@@ -17,6 +17,11 @@ to gather stats from the [Engine API](https://docs.docker.com/engine/api/v1.20/)
   ##   To use environment variables (ie, docker-machine), set endpoint = "ENV"
   endpoint = "unix:///var/run/docker.sock"
 
+  ## Set to true to collect Swarm metrics(desired_replicas, running_replicas)
+  ## Note: configure this in one of the manager nodes in a Swarm cluster.
+  ## configuring in multiple Swarm managers results in duplication of metrics.
+  gather_services = false
+
   ## Only collect metrics for these containers. Values will be appended to
   ## container_name_include.
   ## Deprecated (1.4.0), use container_name_include
@@ -161,6 +166,9 @@ based on the availability of per-cpu stats on your system.
     - available
     - total
     - used
+- docker_swarm
+    - tasks_desired
+    - tasks_running
 
 
 ### Tags:
@@ -191,6 +199,10 @@ based on the availability of per-cpu stats on your system.
     - network
 - docker_container_blkio specific:
     - device
+- docker_swarm specific:
+    - service_id
+    - service_name
+    - service_mode
 
 ### Example Output:
 
@@ -242,4 +254,7 @@ io_service_bytes_recursive_sync=77824i,io_service_bytes_recursive_total=80293888
 io_service_bytes_recursive_write=368640i,io_serviced_recursive_async=6562i,\
 io_serviced_recursive_read=6492i,io_serviced_recursive_sync=37i,\
 io_serviced_recursive_total=6599i,io_serviced_recursive_write=107i 1453409536840126713
-```
+>docker_swarm,
+service_id=xaup2o9krw36j2dy1mjx1arjw,service_mode=replicated,service_name=test,\
+tasks_desired=3,tasks_running=3 1508968160000000000
+```