Add documentation for change in CMAC self-test behaviour

Signed-off-by: Steven Cooreman <steven.cooreman@silabs.com>
diff --git a/ChangeLog.d/allow_alt_cmac_without_des.txt b/ChangeLog.d/allow_alt_cmac_without_des.txt
new file mode 100644
index 0000000..5193a9e
--- /dev/null
+++ b/ChangeLog.d/allow_alt_cmac_without_des.txt
@@ -0,0 +1,3 @@
+Changes
+   * Alternative implementations of CMAC may now opt to not support 3DES as a
+     CMAC block cipher, and still pass the CMAC self test.