readme: document the purpose of the evolve extension Another place for the explanation to live.

prune: replace --biject with --pair People have been complaining about `--biject` being to obscure for a while. A recent discussion with Pulkit Goyal and Dan Villiom Podlaski Christiansen produce some new concrete proposal that we liked. We eventually settle on using `--pair`. The `--biject` flag will be kept as an alias for a while.