doc/design: note on the image trailer size
doc/design: note on the image trailer size

Image trailer might consume notable amount of image slot size.
This path adds note for emphasize that fact.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,ATmobica/mcuboot