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>