[U-Boot] [U-Boot, V9, 3/4] rockchip:usb: add a simple readme for rockusb

[Philipp Tomsich]

> add a simple readme to introduce rockusb and tell people how to use it
> 
> Signed-off-by: Eddie Cai <eddie.cai.linux at gmail.com>
> Reviewed-by: Simon Glass <sjg at chromium.org>
> 
> Changes in v9:                                                                  
> -none
> 
> Changes in v8:                                                                  
> -none
> 
> Changes in v7:
> -none
> 
> Changes in v6:
> -none
> 
> Changes in v5:
> -none
> 
> Changes in v4:
> -add some blank line to make it look better
> 
> Changes in v3:
> -fix checkpatch error
> ---
>  doc/README.rockusb | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 51 insertions(+)
>  create mode 100644 doc/README.rockusb
> 

Acked-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>