docs: release: Add note on firmware loader mode

Adds a note on the new firmware loader operation type

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/docs/release-notes.d/zephyr-firmware-loader.md b/docs/release-notes.d/zephyr-firmware-loader.md
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index 0000000..8f6acf4
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+- Added firmware loader configuration type support for Zephyr, this
+  allows for a single application slot and firmware loader image in
+  the secondary slot which is used to update the primary image
+  (loading it in any way it sees fit e.g. via Bluetooth).