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: NJosh Wu <josh.wu@atmel.com>
Reviewed-by: NJagannadha Sutradharudu Teki <jaganna@xilinx.com>

README

@@ -4069,6 +4069,43 @@ to save the current settings.
 	  environment area within the total memory of your DataFlash placed
 	  at the specified address.
 
+- CONFIG_ENV_IS_IN_SPI_FLASH:
+
+	Define this if you have a SPI Flash memory device which you
+	want to use for the environment.
+
+	- CONFIG_ENV_OFFSET:
+	- CONFIG_ENV_SIZE:
+
+	  These two #defines specify the offset and size of the
+	  environment area within the SPI Flash. CONFIG_ENV_OFFSET must be
+	  aligned to an erase sector boundary.
+
+	- CONFIG_ENV_SECT_SIZE:
+
+	  Define the SPI flash's sector size.
+
+	- CONFIG_ENV_OFFSET_REDUND (optional):
+
+	  This setting describes a second storage area of CONFIG_ENV_SIZE
+	  size used to hold a redundant copy of the environment data, so
+	  that there is a valid backup copy in case there is a power failure
+	  during a "saveenv" operation. CONFIG_ENV_OFFSET_RENDUND must be
+	  aligned to an erase sector boundary.
+
+	- CONFIG_ENV_SPI_BUS (optional):
+	- CONFIG_ENV_SPI_CS (optional):
+
+	  Define the SPI bus and chip select. If not defined they will be 0.
+
+	- CONFIG_ENV_SPI_MAX_HZ (optional):
+
+	  Define the SPI max work clock. If not defined then use 1MHz.
+
+	- CONFIG_ENV_SPI_MODE (optional):
+
+	  Define the SPI work mode. If not defined then use SPI_MODE_3.
+
 - CONFIG_ENV_IS_IN_REMOTE:
 
 	Define this if you have a remote memory space which you