Unclarity about "reference data text files" in INSTALL.md.

[kfogel]

INSTALL.md says:

- Adjust location of reference data text files. This directory is
  used to write reference data files.

        //Reference data outputDirectory
        $conf['check_book']['ref_data_dir'] = 'refdata';

What does this mean? The user won't know; we should explain it to them.