Bump version to 0.4.0 and update documentation

docs/API.md

@@ -593,3 +593,46 @@ Alternative request:
     {
       "slot": 1
     }
+
+
+---
+
+## Version 0.4.0 Notes
+
+The WebUI Config tab now exposes the preferred setup workflow.
+
+Important endpoints used by the WebUI:
+
+    GET  /status
+    GET  /config
+
+    POST /config/device
+    POST /config/relay
+    POST /config/temp
+    POST /config/bms
+    POST /config/save
+    POST /config/factory-reset
+
+    POST /relay/set
+
+    POST /temps/scan
+    POST /temps/assign
+    POST /temps/clear
+
+    POST /config/wifi
+    POST /wifi/connect
+    POST /wifi/clear
+
+    POST /bms/reconnect
+    POST /system/restart
+
+The preferred relay command endpoint remains:
+
+    POST /relay/set
+
+Request:
+
+    {
+      "id": "relay_1",
+      "state": true
+    }