Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
Can we fold 20 into 19 by changing 19 to be:

+| 19     | A `Publisher` MUST produce the same elements, starting with the oldest element still available, in the same sequence for all its subscribers and MAY produce the stream elements at (temporarily) differing rates to different subscribers |

(given that a new Subscription (20) is a subscriber (19))