doc: add doc for adding new models into nightly-ci (#11443)

Co-authored-by: Lianmin Zheng <lianminzheng@gmail.com>

test/README.md

@@ -41,3 +41,9 @@ python3 test_choices.py
 - Give tests descriptive names reflecting their purpose.
 - Use robust assertions (e.g., assert, unittest methods) to validate outcomes.
 - Clean up resources to avoid side effects and preserve test independence.
+- Reduce the test time by using smaller models and reusing the server for multiple test cases.
+
+
+## Adding New Models to Nightly CI
+- **For text models**: extend [global model lists variables](https://github.com/sgl-project/sglang/blob/85c1f7937781199203b38bb46325a2840f353a04/python/sglang/test/test_utils.py#L104) in `test_utils.py`, or add more model lists
+- **For vlms**: extend global variable of lauch setttings list containing `ModelLaunchSettings` in `test_nightly_vlms_.*.py`, see [here](https://github.com/sgl-project/sglang/blob/85c1f7937781199203b38bb46325a2840f353a04/test/srt/test_nightly_vlms_mmmu_eval.py#L18)