Fix readme tarball

Trying again with something extremely simple, this time hopefully on the right branch:

When I took my first stab at using ./platform.sh, I thought the bin-dist parameter was the path to an installed GHC bindist. After it became clear that it actually needed to be the original tarball, it still didn't work, because I had the `xz` tarball, not the `bz2` tarball. I clarified those points in the README.