Add README.md

[tetsuo-cpp]

Summary

The project currently has no top-level README.md. Add one to help new contributors and users understand the project quickly.

Suggested content

• What is WarpForth? — Brief description: an MLIR-based compiler for the Forth programming language targeting GPU kernels.
• Requirements — LLVM/MLIR, CMake, C++17, CUDA (for GPU execution), uv (for Python tooling).
• Build instructions — How to configure CMake and build with cmake --build build.
• Usage examples — How to compile a Forth file to PTX and run it on a GPU, covering the three tools: warpforth-translate, warpforth-opt, warpforth-runner.
• Supported Forth words — Stack ops, arithmetic, bitwise, comparison, control flow, memory, GPU indexing words.
• Architecture overview — Dialect → MemRef → GPU → PTX pipeline.
• Testing — How to run unit tests (check-warpforth) and GPU integration tests (gpu_test/).
• Contributing — Code style (LLVM naming, clang-format), how to add new operations.

Acceptance criteria

• README.md exists at the repository root
• Covers at minimum: what the project is, how to build it, and a quick-start usage example