Rewrite README: explain the shim, current state, and how to adapt for other models

README.md

@@ -1,7 +1,52 @@
-# vLLM to SGLang
+# vLLM → SGLang Shim
 
-vLLM production stack is great, but some things just work out of the box on sglang. 
+Drop-in replacement that makes a vLLM production stack (e.g. the [k8s operator](https://github.com/vllm-project/production-stack)) actually run [SGLang](https://github.com/sgl-project/sglang) instead.
 
-this just kind of just proxies the bootstrapping of a model deployment in vllm stack to sglang
+## Why?
 
-this is a PoC im trying out right now. 
+The vLLM production stack handles model lifecycle, scaling, and routing — but some models work better (or only work) on SGLang. Rather than rewriting your deployment infra, this shim intercepts every vLLM invocation and launches SGLang with equivalent arguments.
+
+## How It Works
+
+Two interception paths:
+
+| What the stack calls | What happens |
+|---|---|
+| `vllm serve <model> [flags]` | Shell shim (`vllm-shim.sh`) parses args, execs `python -m sglang.launch_server` |
+| `python -m vllm.entrypoints.openai.api_server` | Python shim (shadow module on `PYTHONPATH`) does the same |
+
+Both extract `--host` and `--port` from whatever the stack sends and forward them to SGLang. Everything else is currently hardcoded for the target model.
+
+## Current State
+
+**PoC — hardcoded for `mistralai/Devstral-2-123B-Instruct-2512` on 8× MI300X.**
+
+- Model path, `--tp 8`, and `--tool-call-parser mistral` are baked into both shims
+- The Dockerfile builds on `lmsysorg/sglang-rocm` and patches a broken `aiter` build from the base image
+- MI300X tuning env vars are set (`HIP_FORCE_DEV_KERNARG`, `NCCL_MIN_NCHANNELS`, etc.)
+
+## Building
+
+```bash
+docker build -t vllm-to-sglang .
+```
+
+Then use this image anywhere the vLLM stack expects its server image.
+
+## Making It Work For Other Models
+
+Right now the model config is hardcoded in three places:
+
+- `vllm-shim.sh` — the `exec python -m sglang.launch_server` line
+- `vllm_shim_module.py` — the `os.execvp()` call
+- `Dockerfile` — base image and ROCm-specific patches
+
+To adapt for a different model, change `--model-path`, `--tp`, and `--tool-call-parser` in both shim files. A future pass will make this configurable via env vars or args so you don't have to edit source.
+
+## Files
+
+| File | Purpose |
+|---|---|
+| `Dockerfile` | Builds the image: ROCm SGLang base + aiter fix + shims + MI300X env |
+| `vllm-shim.sh` | Shell shim — replaces the `vllm` binary |
+| `vllm_shim_module.py` | Python shim — shadows `vllm.*` module imports |