added readme

Signed-off-by: Jakub Delicat <[email protected]>

ROS_API.md

@@ -11,7 +11,7 @@ Use `bringup.launch.py` from `rosbot_bringup` to start all base functionalities
   - `/odometry/filtered` (_nav_msgs/Odometry_)
 
 
-- `controller.launch.py` from `rosbot_controller`, it loads robot model defined in `rosbot_description` as well as ros2 control [rosbot_hardware_interfaces](https://github.com/husarion/rosbot_hardware_interfaces). It also starts controllers:
+Use `controller.launch.py` from `rosbot_controller`, it loads robot model defined in `rosbot_description` as well as ros2 control [rosbot_hardware_interfaces](https://github.com/husarion/rosbot_hardware_interfaces). It also starts controllers:
   * `joint_state_broadcaster`
   * `rosbot_base_controller`
   * `imu_broadcaster`
@@ -29,5 +29,16 @@ Use `bringup.launch.py` from `rosbot_bringup` to start all base functionalities
   - `/imu_broadcaster/imu` (_sensor_msgs/Imu_)
 
 Use `simulation.launch.py` from `rosbot_gazebo` to start all base functionalities for ROSbot 2, 2 PRO, 2R in the Gazebo simulator.
+If you want to spawn multiple robots use `multirobot_simulation.launch.py` with the `robots` argument e. g.:
+```bash
+ros2 launch rosbot_gazebo multirobot_simulation.launch.py robots:="robot1={y: 0.0}; robot2={y: 1.0}; robot3={y: 2.0}; robot4={y: 3.0}"
+```
+
+If you want to use your own world add to the world's sdf file gazebo sensors plugins inside any `<model>` tag:
+```xml
+<plugin filename="ignition-gazebo-imu-system" name="gz::sim::systems::Imu"/>
+<plugin filename="ignition-gazebo-sensors-system" name="gz::sim::systems::Sensors"/>
+```
+
 > **Warning**
 > The distance sensors' topics types from Gazebo simulation mismatch with the real ones. The range sensors are not implemented yet in the Gazebo Ignition (for more information look [here](https://github.com/gazebosim/gz-sensors/issues/19)). The real type is [sensor_msgs/msg/Range](https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/Range.msg) but simulated [sensor_msgs/msg/LaserScan](https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/LaserScan.msg). The first value of the `ranges` in [sensor_msgs/msg/LaserScan](https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/LaserScan.msg) is the `range` field of [sensor_msgs/msg/Range](https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/Range.msg).