README: Describe CONFIG_SYS_NO_FLASH

Unlike most configuration options defining this actually disables
support for a feature (parallel flash). Eventually the logic behind this
should probably be flipped so that '#ifndef CONFIG_SYS_NO_FLASH' becomes
'#ifdef CONFIG_HAS_PARALLEL_FLASH' but for now lets document the
existing behaviour.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>

README

@@ -3037,6 +3037,19 @@ CBFS (Coreboot Filesystem) support
 		this is instead controlled by the value of
 		/config/load-environment.
 
+- Parallel Flash support:
+		CONFIG_SYS_NO_FLASH
+
+		Traditionally U-boot was run on systems with parallel NOR
+		flash. This option is used to disable support for parallel NOR
+		flash. This option should be defined if the board does not have
+		parallel flash.
+
+		If this option is not defined one of the generic flash drivers
+		(e.g.  CONFIG_FLASH_CFI_DRIVER or CONFIG_ST_SMI) must be
+		selected or the board must provide an implementation of the
+		flash API (see include/flash.h).
+
 - DataFlash Support:
 		CONFIG_HAS_DATAFLASH