README: document CONFIG_ENV_IS_IN_SPI_FLASH

The option can be used to save the environment in spi flash.
Implementation code is already exist in command/env_sf.c. But
the documentation is missing.

This patch add the details for this option to the README file.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>