update doc for client-sdk testing (#849)

As title


## Before submitting

- [ ] This PR fixes a typo or improves the docs (you can dismiss the
other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor
guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

tests/client-sdk/README.md

@@ -0,0 +1,21 @@
+# Llama Stack Integration Tests
+You can run llama stack integration tests on either a Llama Stack Library or a Llama Stack endpoint.
+
+To test on a Llama Stack library with certain configuration, run
+```bash
+LLAMA_STACK_CONFIG=./llama_stack/templates/cerebras/run.yaml
+pytest -s -v tests/client-sdk/inference/test_inference.py
+```
+
+To test on a Llama Stack endpoint, run
+```bash
+LLAMA_STACK_BASE_URL=http//localhost:8089
+pytest -s -v tests/client-sdk/inference/test_inference.py
+```
+
+
+## Common options
+Depending on the API, there are custom options enabled
+- For tests in `inference/` and `agents/, we support `--inference-model` (to be used in text inference tests) and `--vision-inference-model` (only used in image inference tests) overrides
+- For tests in `vector_io/`, we support `--embedding-model` override
+- For tests in `safety/`, we support `--safety-shield` override