Add Checksum for downloads

[bexelbie]

I had a failed install and the online suggestion was bad flash of the USB. It'd be nice to be able to verify my image so I can rule out bad download.

See: https://discussion.fedoraproject.org/t/installation-of-f29-silverblue-fatal-error/647

[b-harper]

In addition to adding the checksum, a link to the torrent would also be nice.

@bexelbie, here is the checksum from the torrent:

console output

$ cat Fedora-Silverblue-29-1.2-x86_64-CHECKSUM 
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

# Fedora-Silverblue-ostree-x86_64-29-1.2.iso: 2112880640 bytes
SHA256 (Fedora-Silverblue-ostree-x86_64-29-1.2.iso) = 0167e4c08b188c623023e0eac259e29a46450cabb3dc1a495cf62ae249360129
-----BEGIN PGP SIGNATURE-----
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=+lbn
-----END PGP SIGNATURE-----

$ sha256sum Fedora-Silverblue-ostree-x86_64-29-1.2.iso 
0167e4c08b188c623023e0eac259e29a46450cabb3dc1a495cf62ae249360129 Fedora-Silverblue-ostree-x86_64-29-1.2.iso