DSMR: Add deprecation warning for YAML configuration (#52179)

homeassistant/components/dsmr/sensor.py

@@ -64,6 +64,12 @@ async def async_setup_platform(
     discovery_info: dict[str, Any] | None = None,
 ) -> None:
     """Import the platform into a config entry."""
+    LOGGER.warning(
+        "Configuration of the DSMR platform in YAML is deprecated and will be "
+        "removed in Home Assistant 2021.9; Your existing configuration "
+        "has been imported into the UI automatically and can be safely removed "
+        "from your configuration.yaml file"
+    )
     hass.async_create_task(
         hass.config_entries.flow.async_init(
             DOMAIN, context={"source": SOURCE_IMPORT}, data=config