Flesh out README (#48)

[dbqpdb]

Summary

Expands the one-line README into a project README that covers:

• What RLM is and its design ethos (kept the original playful tone).
• Installation via conda env create -f environment.yml, with a forward-reference to Refactor: reconsider numpy and pandas dependencies #39 (dependency trimming).
• How to play a game.
• How to run tests (with a note about the in-flight pytest migration Unit testing #23).
• Contributing notes — PR-only workflow, CI expectations.
• Roadmap pointing at concrete tracking issues.
• Non-goals (playing well; silently).

Notes

• References issues that are already filed; doesn't reference any PR numbers since they're less stable.
• Mentions pytest as the going-forward test runner. Strictly speaking pytest isn't on master until Test infra: pytest migration + tests / lint workflows (#23, #43, #44) #50 lands, but the README is forward-looking and the migration is in flight.
• The "PEP 8 work in progress" line in Contributing softens what the lint workflow will surface on incoming PRs.

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