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>