README.md: add dependency on libncurses

This seems to be needed for host-erlang
Run resize2fs to reduce the rootfs partition significantly

This change removes the slack that had been needed to make up for not
being able to pre-calculate the number of blocks required for an image.
For the minimal image, it reduces the size from 16 MB to a little over
13 MB, so it will probably be felt in SDCard programming time. I think
that bigger images will usually be more significantly impacted.