README: add cmd directory description

All of the command files have moved to cmd directory, add description to
Directory Hierarchy.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index 30d7ee3..f41a6af 100644
--- a/README
+++ b/README
@@ -151,6 +151,7 @@
   /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