docs: release-notes: Add note on Zephyr boot banner

Adds a note about the new Zephyr MCUboot boot banner feature

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

docs/release-notes.d/zephyr-boot-banner.md

@@ -0,0 +1,4 @@
+- Zephyr: Added a MCUboot banner which displays the version of
+  MCUboot being used and the version of zephyr. This can be
+  disabled by setting ``CONFIG_MCUBOOT_BOOT_BANNER=n`` which
+  will revert back to the default zephyr boot banner.