docs/guides/redundancy.md: add note on link unavailibility

Signed-off-by: Filip Lewiński <filip.lewinski@3mdeb.com>

Author: filipleple

docs/guides/redundancy.md

@@ -50,4 +50,6 @@ reboot, the system will boot from the newly updated slot.
 ## Further reading
 
 For a more detailed documentation of the implementation, please read the
-[upstream coreboot documentation](https://doc.coreboot.org/soc/intel/redundancy.html).
+[upstream coreboot documentation](https://doc.coreboot.org/soc/intel/redundancy.html)
+(not yet available as of February 24, can also be viewed
+[here](https://github.com/coreboot/coreboot/blob/main/Documentation/soc/intel/redundancy.md))