rockchip: rk3188: add README.rockchip paragraph describing sd boot

Building sd images for rk3188 requires more steps due to the needed split
into TPL and SPL as loaders. Describe how to build an image for it in a
separate paragraph in the READER.rockchip file.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>