imx: Document fuse assignments for MAC addresses

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
diff --git a/doc/README.imx5 b/doc/README.imx5
index e08941e..c5312b6 100644
--- a/doc/README.imx5
+++ b/doc/README.imx5
@@ -20,3 +20,9 @@
     This option should be enabled for boards having a SYS_ON_OFF_CTL signal
     connected to GPIO1[23] and triggering the MAIN_PWR_ON signal like in the
     reference designs.
+
+2. CONVENTIONS FOR FUSE ASSIGNMENTS
+-----------------------------------
+
+2.1 MAC Address: It is stored in the words 9 to 14 of fuse bank 1, using the
+    natural MAC byte order (i.e. MSB first).