Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
I understand those nuances, but nothing about this wording is clear, particularly if we start talking about SPSC/MPMC relationships.

Is this point talking about a `Subscriber` invoking `Subscription.request` and `Subscription.cancel`? 

If so, section 3.5 requires "thread safety" and section 3.2 permits both synchronous and asynchronous invocation.

I'm still confused as to what this is protecting against and what it is saying a `Subscriber` implementation must do.