Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
Why is this up to the `Publisher` to enforce this? That requires bookkeeping in an aggregated data structure whereas normally a unicast `Publisher`/`Subscription` will only ever know about the single `Subscriber` it is currently associated with. The `Publisher` shouldn't have to track every `Subscriber` it ever saw and then deal with the cleanup. This adds concurrency and memory overhead.