docs(autoware_v2x_msgs): add readme

Signed-off-by: Takagi, Isamu <isamu.takagi@tier4.jp>

Author: isamu-takagi

autoware_v2x_msgs/README.md

@@ -2,4 +2,26 @@
 
 ## Virtual gate messages
 
-WIP
+### Overview
+
+This message represents the status of the virtual gate for passing through the area managed by the facility.
+The virtual gate treats area entry permission as a shared resource and controls vehicles by acquiring and releasing locks.
+Each facility may support different protocols, but Autoware V2X component converts each protocol and this message.
+This allows Autoware to handle facilities with different protocols with a unified message.
+
+![virtual-gate-nodes](./doc/virtual-gate-nodes.drawio.svg)
+
+### Sequence
+
+Because there is a time lag before commands are reflected, the vehicle must wait until it receives the status of the same sequence as the command it sent.
+Until the vehicle receives the status, plsease treat it as if it were unlocked.
+
+### Gates
+
+Specify the entrance and exit gate IDs. This is used to check if vehicles can pass simultaneously when multiple routes are possible within an area.
+If omitted, it is treated as a lock for the entire area.
+
+### Vehicle ID
+
+This message does not include the vehicle ID, so add it in the V2X component if required by the communication protocol.
+And if facilities publish multiple vehicle statuses, filter to only status for own vehicle.