Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
The rule only expresses that if the Publisher actually (for whatever reason) still has a previous element available then it is obliged to hand that out to new subscribers. Therefore the rule is not problematic in any way, but it also does not really have a measurable effect, so I would vote to remove it as well.

Another angle on this is that rule 19 could be misinterpreted in such a way that “cold Observables” are in violation of the spec (since they produce outdated elements to some late Subscribers while being much further advanced in the stream for some old Subscribers).