[diffusion] doc: add contributing.md

Author: mickqian

python/sglang/multimodal_gen/docs/contributing.md

@@ -0,0 +1,56 @@
+# 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**: generate the Markdown table.
+    ```bash
+    python python/sglang/multimodal_gen/benchmarks/compare_perf.py baseline.json new.json
+    ```
+
+4.  **Paste**: Copy the output Markdown table into your PR description.
+
+### Example PR Description
+
+```markdown
+### Motivation
+Optimize the VAE decode stage by removing redundant GPU syncs
+
+
+### Performance Impact
+${your_generated_report}
+```