README: add cmd directory description

All of the command files have moved to cmd directory, add description to
Directory Hierarchy.
Signed-off-by: NZiyuan Xu <xzy.xu@rock-chips.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>

README

@@ -151,6 +151,7 @@ Directory Hierarchy:
   /x86			Files generic to x86 architecture
 /api			Machine/arch independent API for external apps
 /board			Board dependent files
+/cmd			U-Boot commands functions
 /common			Misc architecture independent functions
 /configs		Board default configuration files
 /disk			Code for disk drive partition handling