How can I start using fts in attachments in dovecot using decode2text.sh?

Question

I want to index the attachments using the "fts_decoder" plugin of dovecot. The documentation says the setting for same is

plugin {
  fts_decoder = decode2text
}

service decode2text {
  executable = script /usr/lib/dovecot/decode2text.sh
  user = vmail
  unix_listener decode2text {
    mode = 0666
  }
}

But where do I enter this setting or should I create a new file in conf.d folder. Also what are other changes which need to be made for making this to work. I am really new to dovecot so please try to be elaborate.