Document update procedure and limitations

I didn't find a recommended update procedure for the devkit (but might have missed it). I think we should document that like use 'apt dist-upgrade' and notify about the limitation;

As for `apt dist-upgrade`: That will work for everything *besides* kernel and *uboot* since those are not uploaded into the archve yet: (https://source.puri.sm/Librem5/image-builder/issues/18, https://source.puri.sm/Librem5/image-builder/issues/4, https://source.puri.sm/Librem5/image-builder/issues/3)

Maybe conclude: safest is to reflash.