x86: doc: Document coreboot framebuffer driver issue on QEMU

For some unknown reason, coreboot framebuffer driver never works on
QEMU since day 1. It seems the driver only works on real hardware.
Document this issue.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/README.x86

@@ -385,6 +385,9 @@ At present it seems that for Minnowboard Max, coreboot does not pass through
 the video information correctly (it always says the resolution is 0x0). This
 works correctly for link though.
 
+Note: coreboot framebuffer driver does not work on QEMU. The reason is unknown
+at this point. Patches are welcome if you figure out anything wrong.
+
 Test with QEMU for bare mode
 ----------------------------
 QEMU is a fancy emulator that can enable us to test U-Boot without access to