Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
The spec’s mandate is to define the limits which confine implementation semantics, and as such it should not needlessly disallow valid behavior. Since there are Publishers for which no resources are needed (beyond the GC-managed object itself), I would still prefer to turn it around as I argued above; I’ll paste the text here for more easy reading since I messed up the formatting above:

> While the `Subscription` is not canceled, invoking `Subscription.cancel` MAY cause the `Publisher` to transition into the shut-down state if no other `Subscription` exists at this point.