docs: release: Add release notes on changes

Adds release notes on bootutil changes

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

Author: nordicjm

docs/release-notes.d/bootutil-sector.md

@@ -0,0 +1,7 @@
+- bootutil: Fixed issue with comparing sector sizes for
+  compatibility, this now also checks against the number of usable
+  sectors (which is the slot size minus the swap status and moved
+  up by one sector).
+- bootutil: Added debug logging to show write location of swap status
+  and details on sectors including if slot sizes are not optimal for
+  a given board.