README: open with what the split allows and what was measured

[tactino]

The README's opening still led with "the reinforcement learning result is negative" and E1's disproved premise, and counted twenty-eight experiments. This brings it to the positioning the site (plugrl.github.io#20) and the org page (PlugRL/.github#6) now have.

• It opens with what the split allows: env clients in another process, on another machine, on a machine with no GPU, or in a program that is not Python.
• The table lists what backs that: the coverage grid, E2, E12/E13, E7, E10 and E11.
• E1 and E6 leave the table. E1 stays in experiments/README.md, where the research record lives.
• pi0.5's RL fine-tuning record is linked at plugrl.github.io/vla/ rather than headlined.
• The count is now forty experiments, thirty-nine run. E3 was never run.

Only README.md's opening changes.