Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
@benjchristensen This is just a clarification that regardless of how the Subscriber chooses to schedule the processing of an `onNext` signal, it must make sure that that obeys causality (by involving the appropriate rules of the JMM). This is necessary because the Publisher alone has no way to guarantee safe publication of the passed element if that contains non-final fields; this has been discussed many times on the concurrency-interest mailing list, and the situation will not improve until the JVM exposes proper memory barrier primitives.