docs: release-notes: Add note on zephyr auto max sectors

Adds a note about the new feature that has been added

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

docs/release-notes.d/zephyr-auto-max-sectors.md

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+- Added support for automatically calculating the maximum number
+  of sectors that are needed for a build by checking the erase
+  sizes of the partitions using CMake for Zephyr. This behaviour
+  can be reverted to the old manual behaviour by disabling
+  ``CONFIG_BOOT_MAX_IMG_SECTORS_AUTO``