README: document CONFIG_ENV_IS_IN_MMC

Describe the meaning of CONFIG_ENV_IS_IN_MMC, and all related defines that
must or can be set when using that option.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Tom Rini <trini@ti.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>

README

@@ -3645,6 +3645,46 @@ but it can not erase, write this NOR flash by SRIO or PCIE interface.
 	  You will probably want to define these to avoid a really noisy system
 	  when storing the env in UBI.
 
+- CONFIG_ENV_IS_IN_MMC:
+
+	Define this if you have an MMC device which you want to use for the
+	environment.
+
+	- CONFIG_SYS_MMC_ENV_DEV:
+
+	  Specifies which MMC device the environment is stored in.
+
+	- CONFIG_SYS_MMC_ENV_PART (optional):
+
+	  Specifies which MMC partition the environment is stored in. If not
+	  set, defaults to partition 0, the user area. Common values might be
+	  1 (first MMC boot partition), 2 (second MMC boot partition).
+
+	- CONFIG_ENV_OFFSET:
+	- CONFIG_ENV_SIZE:
+
+	  These two #defines specify the offset and size of the environment
+	  area within the specified MMC device.
+
+	  These two values are in units of bytes, but must be aligned to an
+	  MMC sector boundary.
+
+	- CONFIG_ENV_OFFSET_REDUND (optional):
+
+	  Specifies a second storage area, of CONFIG_ENV_SIZE size, used to
+	  hold a redundant copy of the environment data. This provides a
+	  valid backup copy in case the other copy is corrupted, e.g. due
+	  to a power failure during a "saveenv" operation.
+
+	  This value is also in units of bytes, but must also be aligned to
+	  an MMC sector boundary.
+
+	- CONFIG_ENV_SIZE_REDUND (optional):
+
+	  This value need not be set, even when CONFIG_ENV_OFFSET_REDUND is
+	  set. If this value is set, it must be set to the same value as
+	  CONFIG_ENV_SIZE.
+
 - CONFIG_SYS_SPI_INIT_OFFSET
 
 	Defines offset to the initial SPI buffer area in DPRAM. The