docs: add readme (autowarefoundation#756) (autowarefoundation#135)

Signed-off-by: kosuke murakami <kosuke.murakami@tier4.jp>
Signed-off-by: wep21 <border_goldenmarket@yahoo.co.jp>

Co-authored-by: Kosuke Murakami <kosuke.murakami@tier4.jp>
Co-authored-by: Takeshi Miura <57553950+1222-takeshi@users.noreply.github.com>

control/external_cmd_selector/README.md

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+# external_cmd_selector
+
+## Purpose
+
+`external_cmd_selector` is the package to publish `external_control_cmd`, `gear_cmd`, `hazard_lights_cmd`, `heartbeat` and `turn_indicators_cmd`, according to the current mode, which is `remote` or `local`.
+
+The current mode is set via service, `remote` is remotely operated, `local` is to use the values calculated by Autoware.
+
+## Input / Output
+
+### Input topics
+
+| Name                                           | Type | Description                                             |
+| ---------------------------------------------- | ---- | ------------------------------------------------------- |
+| `/api/external/set/command/local/control`      | TBD  | Local. Calculated control value.                        |
+| `/api/external/set/command/local/heartbeat`    | TBD  | Local. Heartbeat.                                       |
+| `/api/external/set/command/local/shift`        | TBD  | Local. Gear shift like drive, rear and etc.             |
+| `/api/external/set/command/local/turn_signal`  | TBD  | Local. Turn signal like left turn, right turn and etc.  |
+| `/api/external/set/command/remote/control`     | TBD  | Remote. Calculated control value.                       |
+| `/api/external/set/command/remote/heartbeat`   | TBD  | Remote. Heartbeat.                                      |
+| `/api/external/set/command/remote/shift`       | TBD  | Remote. Gear shift like drive, rear and etc.            |
+| `/api/external/set/command/remote/turn_signal` | TBD  | Remote. Turn signal like left turn, right turn and etc. |
+
+### Output topics
+
+| Name                                                   | Type                                                   | Description                                     |
+| ------------------------------------------------------ | ------------------------------------------------------ | ----------------------------------------------- |
+| `/control/external_cmd_selector/current_selector_mode` | TBD                                                    | Current selected mode, remote or local.         |
+| `/diagnostics`                                         | diagnostic_msgs::msg::DiagnosticArray                  | Check if node is active or not.                 |
+| `/external/selected/external_control_cmd`              | TBD                                                    | Pass through control command with current mode. |
+| `/external/selected/gear_cmd`                          | autoware_auto_vehicle_msgs::msg::GearCommand           | Pass through gear command with current mode.    |
+| `/external/selected/hazard_lights_cmd`                 | autoware_auto_vehicle_msgs::msg::HazardLightsCommand   | Pass through hazard light with current mode.    |
+| `/external/selected/heartbeat`                         | TBD                                                    | Pass through heartbeat with current mode.       |
+| `/external/selected/turn_indicators_cmd`               | autoware_auto_vehicle_msgs::msg::TurnIndicatorsCommand | Pass through turn indicator with current mode.  |