docs: release: Add note on zephyr debug mode

Adds a note that zephyr builds no longer build in debug mode
by default

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/docs/release-notes.d/zephyr-debug.md b/docs/release-notes.d/zephyr-debug.md
new file mode 100644
index 0000000..71a09f7
--- /dev/null
+++ b/docs/release-notes.d/zephyr-debug.md
@@ -0,0 +1,2 @@
+- Zephyr no longer builds in optimize for debug mode, this saves a
+  significant amount of flash space.