Add note about how to change initial pose

irobot_create_gz/irobot_create_gz_bringup/README.md

@@ -13,4 +13,10 @@ lower-left corner to start the simulation.  The robot starts docked on its charg
 The default simulation world is the `depot` environment. To chage worlds, use the `world`
 argument.  Supported worlds are:
 - `depot` (default)
-- `maze`
+- `maze`
+
+## Robot spawn location
+
+By default the robot spawns at the origin.  To change the robot's spawn location, use the
+`x`, `y`, `z` and `yaw` arguments.  Note that changing `z` may result in the robot spawning
+in the air or below the ground plane, depending in the value used.