Include Game: Synchronous Chess

Try out [Synchronous Chess] and [Simultaneous Chess] to see if either one is playable.

* [x] Try Synchronous. It sounds more promising, with an intermediate phase between synchronous moves where players can exchange blows on the pieces that just moved. There's also a [discussion] of the history and some rules questions.
* [x] Try Simultaneous, although it doesn't seem like a big enough change. White starts with "initiative". Player without initiative loses a turn when moves conflict, but takes initiative.
* [x] Look for more simultaneous movement variants in D.B. Pritchard's [Encyclopedia of Chess Variants].
* [x] Choose one to include, or neither.
* [x] Add to rules.
* [ ] Add example and diagram.

[Synchronous Chess]: http://www.hexenspiel.de/engl/synchronous-chess/
[discussion]: https://www.chessvariants.org/index/listcomments.php?itemid=SynchronousChess&first=7278&last=13089
[Simultaneous Chess]: https://www.chessvariants.com/rules/simultaneous-chess
[Encyclopedia of Chess Variants]: https://web.archive.org/web/20180813210317/https://www.jsbeasley.co.uk/encyc/encyc.pdf