def doi
      # Try README first
      if readme_doi = Metamatter::Readme.new(self).doi
        return readme_doi
      elsif datacite_doi = Metamatter::Datacite.new(self).doi
        return datacite_doi.first
      else
        return nil
      end
    end
def doi
      # Try README first
      if readme_doi = Metamatter::Readme.new(self).doi
        return readme_doi
      elsif datacite_doi = Metamatter::Datacite.new(self).doi
        return datacite_doi.first
      else
        return nil
      end
    end
Public: Returns any known DOIs for the repository

 Returns a DOI string or nil