Document Subscribe to trigger websocket (#1182)

Co-authored-by: Paulus Schoutsen <paulus@home-assistant.io>

docs/api/websocket.md

@@ -192,9 +192,94 @@ For each event that matches, the server will send a message of type `event`. The
 }
 ```
 
+## Subscribe to trigger
+
+You can also subscribe to one or more triggers with `subscribe_trigger`. These are the same triggers syntax as used for [automation triggers](https://www.home-assistant.io/docs/automation/trigger/). You can define one or a list of triggers.
+
+```json
+{
+    "id": 2,
+    "type": "subscribe_trigger",
+    "trigger": {
+        "platform": "state",
+        "entity_id": "binary_sensor.motion_occupancy",
+        "from": "off",
+        "to":"on"
+    },
+}
+```
+
+As a response you get:
+
+```json
+{
+ "id": 2,
+ "type": "result",
+ "success": true,
+ "result": null
+}
+```
+
+For each trigger that matches, the server will send a message of type `trigger`. The `id` in the message will point at the original `id` of the `subscribe_trigger` command. Note that your variables will be different based on the used trigger.
+
+```json
+{
+    "id": 2,
+    "type": "event",
+    "event": {
+        "variables": {
+            "trigger": {
+                "id": "0",
+                "idx": "0",
+                "platform": "state",
+                "entity_id": "binary_sensor.motion_occupancy",
+                "from_state": {
+                    "entity_id": "binary_sensor.motion_occupancy",
+                    "state": "off",
+                    "attributes": {
+                        "device_class": "motion",
+                        "friendly_name": "motion occupancy"
+                    },
+                    "last_changed": "2022-01-09T10:30:37.585143+00:00",
+                    "last_updated": "2022-01-09T10:33:04.388104+00:00",
+                    "context": {
+                        "id": "90e30ad8e6d0c218840478d3c21dd754",
+                        "parent_id": null,
+                        "user_id": null
+                    }
+                },
+                "to_state": {
+                    "entity_id": "binary_sensor.motion_occupancy",
+                    "state": "on",
+                    "attributes": {
+                        "device_class": "motion",
+                        "friendly_name": "motion occupancy"
+                    },
+                    "last_changed": "2022-01-09T10:33:04.391956+00:00",
+                    "last_updated": "2022-01-09T10:33:04.391956+00:00",
+                    "context": {
+                        "id": "9b263f9e4e899819a0515a97f6ddfb47",
+                        "parent_id": null,
+                        "user_id": null
+                    }
+                },
+                "for": null,
+                "attribute": null,
+                "description": "state of binary_sensor.motion_occupancy"
+            }
+        },
+        "context": {
+            "id": "9b263f9e4e899819a0515a97f6ddfb47",
+            "parent_id": null,
+            "user_id": null
+        }
+    }
+}
+```
+
 ### Unsubscribing from events
 
-You can unsubscribe from previously created subscription events. Pass the id of the original subscription command as value to the subscription field.
+You can unsubscribe from previously created subscriptions. Pass the id of the original subscription command as value to the subscription field.
 
 ```json
 {