Document the CONFIG_DISPLAY_BOARDINFO option

Add a short note about this in the README.
Signed-off-by: NSimon Glass <sjg@chromium.org>

README

@@ -3457,6 +3457,11 @@ use the "saveenv" command to store a valid environment.
 		space for already greatly restricted images, including but not
 		limited to NAND_SPL configurations.
 
+- CONFIG_DISPLAY_BOARDINFO
+		Display information about the board that U-Boot is running on
+		when U-Boot starts up. The board function checkboard() is called
+		to do this.
+
 Low Level (hardware related) configuration options:
 ---------------------------------------------------