TableOfContents.md: mention PLFArend as a Derived work

[marat-rkh]

Hello! I have been working on rewriting the PLFA in Arend. @wadler suggested mentioning it as a derived work, thank you for this opportunity!
https://agda.zulipchat.com/#narrow/channel/238741-general
I have added the attribution to the README.md of my project as required by the CC-BY-4.0. Once again, sorry for not doing it in the first place. No bad intentions, I just considered this work to be a personal project that very few people would be interested in.
Also, I have exacted my work to a separate repo (thanks for the PLFArend name idea!). This way it is clearly separated from the original book.

[wenkokke]

Heya! I would suggest you add the version of the book that PLFArend is based in the text you're adding here. I should probably release a new version sometime soon, so let's wait a short while so we can coordinate this. What commit did you fork from?

Have you finished the rewrite? If not, you should add your progress.

[wadler]

Thanks to @marat-rkh for this pull request. @wenkokke I've requested an immediate merge, because I think it is better to have incomplete information than no information. Adding more info (such as a link to the Arend project, and info on the version of PLFA used) is a fine idea, but it's ok if that happens later. This way, at least it is documented, if imperfectly.

[marat-rkh]

@wenkokke My project is based on a commit that precedes v22.08. A quick review of remaining commits show that there haven't been many changes, so I plan to update to v22.08. I will do that and add the version to the project's readme. The progress is already reflected there.

[marat-rkh]

Done, PLFArend is updated to 22.08, note is added to the readme.