Write comprehensive project README

[tgoodwin]

Motivation

• Provide a usable, self-contained project README so new users and contributors can understand and run Monolift.
• Summarize the compiler's workflow and surface the pragma and CLI conventions used by the codebase.
• Point readers to the project demo and to the vision paper for additional context.

Description

• Replace the minimal README.md with a detailed document that explains the project goals, workflow, and capabilities.
• Add a link to the vision paper: https://dl.acm.org/doi/10.1145/3764860.3768327 and a high-level "How it works" sequence.
• Document // @monolift pragma syntax, CLI usage with examples (e.g., make build, ./monolift --dirname ... --manifest ...), and the generated output layout.
• Add pointers to the demo under demo/ and a short repository layout summary.

Testing

• No automated tests were run because this is a documentation-only change.

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Codex Task