Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
Sure, but that means it's acting as a `Subscriber` and has chosen to proactively request data. This is already covered by the contract of `Subscriber`. A `Processor` does not need to "be prepared" to receive elements as if they are going to just start showing up, which is what this sentence makes it sound like can happen.

Item 4.1 already states that it must behave correctly as both a `Subscriber` and `Publisher`. We don't need this sentence and its wording is confusing in that it alludes to data arriving without having requested it.