[U-Boot] README: add cmd directory description

[Tom Rini]

On Fri, Aug 26, 2016 at 07:54:49PM +0800, Xu Ziyuan wrote:

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

Applied to u-boot/master, thanks!

-- 
Tom
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