docs: add ELI5 blurb at top of README

[jkbennitt]

Summary

Add a short, honest ELI5 at the top of README.md (right after the title) so GitHub visitors hit the locked public proof first.

README-only. No HF, site, scoring, or Benchmark Results rewrite. Existing technical lead is unchanged.

What the blurb states

• RLE is a public research benchmark (not a product) where AI agent setups try to keep a RimWorld colony alive.
• Published leaderboard: one 11-model Crashlanded run (2026-06-11, N=1, seed 42, 10 ticks) on the felix harness at scoring 1.1.
• Grok 4.3 leads at mean composite 0.836; content-first, not statistically valid.
• Winners advance to N=4; N=4 is not published (same honesty as the existing Benchmark Results section).
• Those rows are not comparable to scoring 1.2.
• Harness × model + scoring 1.2 is current code/docs, not a published matrix.

One-line cross-link to Benchmark Results. Felix is named only as the harness that produced the published table. No GTM, defense/gov/HIPAA/SOC2/customers/copilot language, and no leftover “7 agents / 10-metric” framing in the blurb.

Test plan

• ELI5 sits immediately under the title on GitHub.
• Numbers match the locked pack: 11-model / Grok 0.836 / felix + scoring 1.1 / N=1 / 1.2 not published.
• Benchmark Results and Scoring sections unchanged.
• CI may be billing-blocked; docs-only change, no code/test impact.