Add doc folder

Author: christophfroehlich

battery_state_broadcaster/doc/userdoc.rst

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+:github_url: https://github.com/ros-controls/ros2_controllers/blob/{REPOS_FILE_BRANCH}/battery_state_broadcaster/doc/userdoc.rst
+
+.. _battery_state_broadcaster_userdoc:
+
+Battery State Broadcaster
+=========================
+This broadcaster publishes `sensor_msgs/BatteryState <https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/BatteryState.msg>`__ messages from appropriate state interfaces.
+
+
+Required State Interfaces
+---------------------------------
+This broadcaster requires the robot to have a sensor component which contains the battery state interfaces:
+
+.. code-block:: xml
+
+  <ros2_control type="system">
+
+      <!-- ... -->
+
+      <sensor name="battery_state">
+          <state_interface name="voltage" />
+      </sensor>
+  </ros2_control>
+
+Parameters
+---------------------------------
+To use this broadcaster, declare it in the controller manager and set its parameters:
+
+.. code-block:: yaml
+
+  controller_manager:
+    ros__parameters:
+      battery_state_broadcaster:
+        type: battery_state_broadcaster/BatteryStateBroadcaster
+
+  battery_state_broadcaster:
+    ros__parameters:
+      sensor_name: "battery_state"
+      design_capacity: 100.0
+      # https://github.com/ros2/common_interfaces/blob/rolling/sensor_msgs/msg/BatteryState.msg
+      power_supply_technology: 2
+
+And spawn it in the launch file:
+
+.. code-block:: python
+
+  battery_state_broadcaster_spawner = Node(
+      package="controller_manager",
+      executable="spawner",
+      arguments=["battery_state_broadcaster"]
+
+Topics
+---------------------------------
+The battery state is published on ``~/battery_state``.
+Since it's a plugin within the controller manager, add a remapping of the form ``("~/battery_state", "/my_battery_state")`` to the *controller manager*, not the spawner, to change the topic name.