The manpage says:

> The  sums  are  computed as described in FIPS-180-2.  When checking, the input should be a former
> output of this program.  The default mode is to print a line with checksum, **a space, a  character
> indicating  input  mode ('*' for binary, ' ' for text or where binary is insignificant),** and name
> for each FILE.

By adding the filename to the generated sha256 file, a user would be
able to download an iso, the sha256 file, then use `sha256 -c
[file].iso.sha256` to verify the file.

* * *

This fixes nixos-homepage#224

Adds filename to sha256 file for use with `sha256 -c`