use retain flag when publishing registration messages

Signed-off-by: Reuben Miller <[email protected]>

docs/src/contribute/design/mqtt-topic-design.md

@@ -121,7 +121,7 @@ graph LR
 The equipment, which is a conveyor belt called "belt01", is located in factory "factory01", in the "hallA" building, in the "packaging" area. The conveyor belt can be registered as a child-device by publishing the following message.
 
 ```sh te2mqtt
-tedge mqtt pub 'te/factory01/hallA/packaging/belt001' '{
+tedge mqtt pub -r 'te/factory01/hallA/packaging/belt001' '{
   "@type": "child-device",
   "displayName": "belt001",
   "type": "ConveyorBelt",
@@ -158,7 +158,7 @@ When applying your own semantics to the **identifier**, you can leave any segmen
 For example, if it does not make sense to have the factory and building in the **identifier**, then they can be removed and the equipment can be registered using:
 
 ```sh te2mqtt
-tedge mqtt pub 'te/packaging/belt001' '{
+tedge mqtt pub -r 'te/packaging/belt001' '{
   "@type": "child-device"
 }'
 ```