image-gui-start: add that a screen must be connected to the board for testing HMI images

Signed-off-by: Dylan Aïssi <dylan.aissi@collabora.com>

test-cases/image-gui-start.yaml

@@ -22,6 +22,7 @@ metadata:
     - "Download the virtual machine image for the latest SDK release from:"
     - "~https://images.apertis.org/"
     - "For HMI/Development images:"
+    - "A screen must be connected to the target"
     - "Download latest hmi/development image released from:"
     - "~https://images.apertis.org/"
     - "Write image to a SD card as explained at:"