docs: release-notes: Add note on new swap using offset mode

Adds a note about this new algorithm being added

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

Author: nordicjm

docs/release-notes.d/swap-using-offset.md

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+- Added a new swap using offset algorithm which is set with
+  `MCUBOOT_SWAP_USING_OFFSET`. This algorithm is similar to swap
+  using move but avoids moving the sectors in the primary slot
+  up by having the update image written in the second sector in
+  the update slot, which offers a faster update process and
+  requires a smaller swap status area