Added castling.club (chess game)

watchlist-for-activitypub-apps.md

@@ -236,6 +236,8 @@ Friendica (see above) has an [events engine that federates events over AP](https
 
 * **[Learn Awesome](https://github.com/learn-awesome/learn)**: Open-source equivalent review aggregation site. Think GoodReads, but generalized to all learning resources organized by topics, formats and difficulty `AGPL-3.0, Ruby`
 
+* :black_nib: **[castling.club](https://github.com/stephank/castling.club)** ([site](https://castling.club/), [Fedi account](https://mastodon.social/@kosinus)): Challenge someone to a game of chess using toots. `MIT, Typescript`
+
 #### Is it Alive?
 
 Projects that seem dormant and may be dead, but may also be doing dev quietly in private, or have moved to a new code forge without leaving a forwarding address, etc. Any information that could help us clarify whether these projects are still alive would be much appreciated.