Doc/mb/lenovo/Sandy_Bridge_series.md: Clarify installation

Unlike Ivy Bridge series, there isn't a method to flash coreboot
internally when running vendor firmware (yet). Until someone finds a way
to bypass flash protections, the first flash has to be done externally.

Change-Id: Idaff264f2b7277516d69d1323f1a0c885b28c3db
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/49848
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/Documentation/mainboard/lenovo/Sandy_Bridge_series.md b/Documentation/mainboard/lenovo/Sandy_Bridge_series.md
index 16f0929..84bae40 100644
--- a/Documentation/mainboard/lenovo/Sandy_Bridge_series.md
+++ b/Documentation/mainboard/lenovo/Sandy_Bridge_series.md
@@ -22,6 +22,10 @@
 ```
 
 ## Installation instructions
+
+Flashing coreboot for the first time needs to be done using an external
+programmer, because vendor firmware prevents rewriting the BIOS region.
+
 * Update the EC firmware, as there's no support for EC updates in coreboot.
 * Do **NOT** accidentally swap pins or power on the board while a SPI flasher
   is connected. It will destroy your device.