README: elaborate on GOPATH setup and where to find u-root
README: elaborate on GOPATH setup and where to find u-root

Signed-off-by: Daniel Maslowski <59bd0a3ff43b32849b319e645d4798d8a5d1e889@orangecms.org>

u-root/u-root,u-root/u-root,u-root/u-root