Add the Clay Programming Language Once It's Documented Publicly #2

     Recent versions of Clay have temporarily remained closed-source while their developer and vendor has felt it hasn't been in a state to share publicly. Once that's no longer the case, I'll revive the 'on-hiatus-or-abandoned/add_read-me_entries_from_Slack/add-Clay-language' branch, on which I've staged the change needed to close this issue, and merge it in.