docs: release-notes: Add note on zephyr cache fix

Adds a note on the fix for zephyr cache support

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
diff --git a/docs/release-notes.d/zephyr-cache.md b/docs/release-notes.d/zephyr-cache.md
new file mode 100644
index 0000000..72cc01b
--- /dev/null
+++ b/docs/release-notes.d/zephyr-cache.md
@@ -0,0 +1,4 @@
+- Zephyr: Fixes support for disabling instruction/data caches prior
+  to chain-loading an application, this will be automatically
+  enabled if one or both of these caches are present. This feature
+  can be disabled by setting `CONFIG_BOOT_DISABLE_CACHES` to `n`.