README.md missing explanation of BoolRing, BoolRng, and GenBool

[johnynek]

We should add them to the tables I guess. It will require a new column for ring/lattice, I think.

[TomasMikula]

For ring-like structures, we could add column "Idempotent *?", which is a defining property of BoolR(i)ng. For lattices, we could add columns "pseudo-complement" (Heyting, Bool), "relative-complement" (GenBool, Bool), "complement" (Bool).