docs: Add note on addition of image state/image set state commands

Adds a note that both of these functions are now (optionally)
supported in serial recovery mode.

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/docs/release-notes.d/img-state.md b/docs/release-notes.d/img-state.md
new file mode 100644
index 0000000..1302be3
--- /dev/null
+++ b/docs/release-notes.d/img-state.md
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+- Serial recovery image state and image set state optional commands added