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>

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

@@ -0,0 +1,8 @@
+- 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
+- Made swap using offset the default algorithm for Zephyr builds
+  that do not have a scratch partition