[U-Boot] [U-Boot, 3/3] rockchip: rk3399: Add more instructions to the README

[Philipp Tomsich]

> This commit adds a content section and also instructions
> on how to create a bootable SD/MMC device for RK3399 boards.
> 
> Signed-off-by: Ezequiel Garcia <ezequiel at collabora.com>
> Reviewed-by: Simon Glass <sjg at chromium.org>
> Reviewed-by: Philipp Tomsich <philipp.tomsich at theobroma-systems.com>
> ---
>  board/rockchip/evb_rk3399/README | 55 ++++++++++++++++++++++++++++++--
>  1 file changed, 53 insertions(+), 2 deletions(-)
> 

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