uboot-imx

README: Add instructions for chain-loading U-Boot

Most Chromebooks support chain-loading U-Boot but instructions are
somewhat scattered. Add a README to hold this information within the
U-Boot tree. Also add the standard developer keys to simplify the
instructions, since they are small.

For now this only supports nyan-big.

Signed-off-by: Simon Glass <sjg@chromium.org>