Add note about DCM format

Author: dominicbetts

articles/iot-central/overview-iot-central-pnp.md

@@ -58,6 +58,7 @@ This article outlines, for Azure IoT Central:
 - Application template export and application copy aren't supported.
 - Only the C device SDK and the Node.js device and service SDKs are supported.
 - It's only available the north Europe and central US regions.
+- Device capability models must have all the interfaces defined inline in the same file.
 
 ## Personas