docs: release-notes: Add note on bootutil changes

Adds notes on the changed bootutil features

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/docs/release-notes.d/bootutil-image-verification.md b/docs/release-notes.d/bootutil-image-verification.md
new file mode 100644
index 0000000..a1cc588
--- /dev/null
+++ b/docs/release-notes.d/bootutil-image-verification.md
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+- Changed bootutil's order of events to verify the image header
+  before checking the image.
+- Added the bootloader state object to the bootutil
+  boot_is_header_valid() function