Book updated!

The book is updated to the newest version, both on GitHub hosted repository, and preview on a Google Drive. Version of the newest update is 20211011.093610, and is compiled on October 11th, 2021.

Changes include: 

• fixed, simplified grammar
• clarified demoting to Pawn
• simplified demoting to Pawn notation
• changed example how Wave activated by Unicorn moves
• tiny edits, fixes

Wave activated by Unicorn now moves as it was moving in its original installment, i.e. as an unrestricted Centaur. Reason why I reverted back on Unicorn-activated Wave is that Wave in this case is limited to only one momentum, so whatever is activated by a Wave, it can do only one step. This change makes Wave activated by Unicorn more useful, even if only in a very specific situations. Also, movement rules are simplified, so it's easier to reason about them.

Centaur, obviously enough, remains restricted in its movement, it's still  overpowered as it is. While it might be possible to restrict it further, I'd prefer simple rules over nerfing it into the ground. OTOH, its movement is kind-a tricky to get it right, and so Centaur being overpowered is, in a sense, a reward for a skilled player.

Having two very similar set of rules, with only one minor but essential distinction, for two related pieces is unfortunate fact of life. I don't think that'll cause major issues. Anyone having a go with both Unicorn and Centaur isn't merely interested in chess, otherwise he (or she) would be satisfied playing classical chess. Repeating rules, and adding pieces which are combination of existing ones, only on a larger chessboard isn't particular fun, scaling things up doesn't add anything new. Hence, different rules.