Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
This conflicts with 2.7 as this is permitting calls to `request` to come from either the same thread (synchronous) or another thread (asynchronous).

I suggest leaving this as is and removing 2.7.

Perhaps we clarify more in this section that `Subscription.cancel` and `Subscription.request` must be thread-safe and be capable of handling invocation from any thread synchronously or asynchronously.