Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
While there certainly are Publishers with the characteristics you describe, this is not in general true: most “hot” sources will have knowledge of whom to produce to in a central place already.

That being said, your point is valid that requiring this behavior of all Publishers is too limiting. What about changing the MUST into a RECOMMENDED, meaning that the Publisher should inform the caller of `subscribe` that they are doing something wrong if possible; we are regulating a spec violation in any case.