[diffusion] doc: add contributing.md (#13649)

python/sglang/multimodal_gen/README.md

@@ -65,7 +65,7 @@ For more usage examples (e.g. OpenAI compatible API, server mode), check [cli.md
 
 ## Contributing
 
-All contributions are welcome.
+All contributions are welcome. The contribution guide is available [here](https://github.com/sgl-project/sglang/tree/main/python/sglang/multimodal_gen/docs/contributing.md).
 
 ## Acknowledgement
 

python/sglang/multimodal_gen/docs/contributing.md

@@ -0,0 +1,46 @@
+# Contributing to SGLang Diffusion
+
+This guide outlines the requirements for contributing to the SGLang Diffusion module (`sglang.multimodal_gen`).
+
+## 1. Commit Message Convention
+
+We follow a structured commit message format to maintain a clean history.
+
+**Format:**
+```text
+[diffusion] <scope>: <subject>
+```
+
+**Examples:**
+- `[diffusion] cli: add --perf-dump-path argument`
+- `[diffusion] scheduler: fix deadlock in batch processing`
+- `[diffusion] model: support Stable Diffusion 3.5`
+
+**Rules:**
+- **Prefix**: Always start with `[diffusion]`.
+- **Scope** (Optional): `cli`, `scheduler`, `model`, `pipeline`, `docs`, etc.
+- **Subject**: Imperative mood, short and clear (e.g., "add feature" not "added feature").
+
+## 2. Performance Reporting
+
+For PRs that impact **latency**, **throughput**, or **memory usage**, you **should** provide a performance comparison report.
+
+### How to Generate a Report
+
+1.  **Baseline**: run the benchmark (for a single generation task)
+    ```bash
+    $ sglang generate --model-path <model> --prompt "A benchmark prompt" --perf-dump-path baseline.json
+    ```
+
+2.  **New**: run the same benchmark, without modifying any server_args or sampling_params
+    ```bash
+    $ sglang generate --model-path <model> --prompt "A benchmark prompt" --perf-dump-path new.json
+    ```
+
+3.  **Compare**: run the compare script, which will print a Markdown table to the console
+    ```bash
+    $ python python/sglang/multimodal_gen/benchmarks/compare_perf.py baseline.json new.json
+    ### Performance Comparison Report
+    ...
+    ```
+4. **Paste**: paste the table into the PR description