Consider adding paper to `identifiers` within `CITATION.cff`

[d33bs]

I noticed that the Parsl paper (https://doi.org/10.1145/3307681.3325400) isn't listed within the CITATION.cff. The file could be enhanced to include through the identifiers block. The project could also take advantage of the preferred-citation block to help clear up any discrepancies between citation desires by the authors.