Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
This rule is the other side of 1.6: having emitted a terminal signal means that the one who did the signaling needs to consider the affected Subscription canceled. This means that the upstream element can shut down and release all of its resources without having to wait for a `cancel` invocation.

@smaldini We agreed previously to remove the “or any other …” part of the sentence, since that does not work for Processors.