Move the modules warning before the build instructions
Move the modules warning before the build instructions

This caused trouble for someone today, as they followed the instructions
in the order shown.

Signed-off-by: Ronald G. Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>

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