Documentation/mainboard/lenovo/t420.md: add pic of chip

Provide pic of the flash IC with pinouts labeled, as well
as additional text about the chip.

Signed-off-by: Michael Bacarella <michael.bacarella@gmail.com>
Change-Id: I9046fa63dcd4d192836417efac68ca7587ac1c91
Signed-off-by: Michael Bacarella <michael.bacarella@gmail.com>
Reviewed-on: https://review.coreboot.org/c/30027
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/mainboard/lenovo/t420.md

@@ -1,8 +1,17 @@
 # Lenovo T420
 
 ## Flashing instructions
-You have to disassemble the whole device, as the flash IC is on the bottom
-of the mainboard.
+The flash IC is located at the bottom center of the mainboard.  Sadly,
+access to the IC is blocked by the magnesum frame, so you need to disassemble
+the entire laptop and remove the mainboard.
+
+Below is a picture of IC on the mainboard, with the pinouts labeled.
+
+[t420_chip_location]: t420_chip_location.jpg
+
+The chip will either be a Macronix MX25L6404E (shown above) or a Winbond
+W25Q64CVSIG.  Do not rely on dots painted in the corner of the chip (such as
+the blue dot pictured) to orient the pins!
 
 For more details have a look at [T420 / T520 / X220 / T420s / W520 common] and