Prepare for 0.4.0.M1

Docs tweaking and package info renaming in README.md
I’d prefer to make forward progress by merging this PR without waiting for the outcome of this discussion; we can still fix things later, and ideally this should be the last “huge” PR that completely rewrites the spec in one go. We should aim for smaller, more compact changes in the future (I’m not saying that this PR is bad in any way, we just need to make sure to stay on the agreed-upon ground in order to converge in a reasonable time frame).