[docs] Add integration test example to copy pasta template (#5961)

Co-authored-by: Julien Chaumond <chaumond@gmail.com>

CONTRIBUTING.md

@@ -215,7 +215,7 @@ Follow these steps to start contributing:
      `RUN_SLOW=1 python -m pytest tests/test_my_new_model.py`. 
    - If you are adding a new tokenizer, write tests, and make sure 
      `RUN_SLOW=1 python -m pytest tests/test_tokenization_{your_model_name}.py` passes.
-   CircleCI does not run the slow tests. 
+   CircleCI does not run the slow tests, but github actions does every night!
 6. All public methods must have informative docstrings that work nicely with sphinx. See `modeling_ctrl.py` for an 
    example.
 
@@ -239,6 +239,16 @@ $ pip install -r examples/requirements.txt  # only needed the first time
 $ python -m pytest -n auto --dist=loadfile -s -v ./examples/
 ```
 
+and for the slow tests:
+
+```bash
+RUN_SLOW=yes python -m pytest -n auto --dist=loadfile -s -v ./tests/
+```
+or 
+```python
+RUN_SLOW=yes python -m pytest -n auto --dist=loadfile -s -v ./tests/
+```
+
 In fact, that's how `make test` and `make test-examples` are implemented!
 
 You can specify a smaller set of tests in order to test only the feature