doc: release-notes: Add mcumgr image header callback addition details
doc: release-notes: Add mcumgr image header callback addition details

Adds information on adding the image header details to the img mgmt
callback for application-level determination of what to allow or
decline.

Signed-off-by: Jamie McCrae <7c977670495befe33b67661379bf81cc6b3660ab@lairdconnect.com>

galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr