README: document CONFIG_CMD_FS_GENERIC

This enables generic filesystem commands such as load and ls, which
automatically work with multiple filesystem types, without having to
be told which is present, unlike e.g. ext2load, fatls.

Signed-off-by: Stephen Warren <swarren@nvidia.com>

README

@@ -923,6 +923,8 @@ The following options need to be configured:
 		CONFIG_CMD_EXPORTENV	* export the environment
 		CONFIG_CMD_EXT2		* ext2 command support
 		CONFIG_CMD_EXT4		* ext4 command support
+		CONFIG_CMD_FS_GENERIC	* filesystem commands (e.g. load, ls)
+					  that work for multiple fs types
 		CONFIG_CMD_SAVEENV	  saveenv
 		CONFIG_CMD_FDC		* Floppy Disk Support
 		CONFIG_CMD_FAT		* FAT command support