Update contribution instructions.

Also provide shortcuts in a Makefile.

docs/source/installation.md

@@ -24,23 +24,7 @@ pip install [--editable] .
 
 An extensive test suite is included to test the library behavior and several examples. Library tests can be found in the [tests folder](https://github.com/huggingface/transformers/tree/master/tests) and examples tests in the [examples folder](https://github.com/huggingface/transformers/tree/master/examples).
 
-Tests can be run using `unittest` or `pytest` (install pytest if needed with `pip install pytest`).
-
-Run all the tests from the root of the cloned repository with the commands:
-
-```bash
-python -m unittest discover -s tests -t . -v
-python -m unittest discover -s examples -t examples -v
-```
-
-or
-
-``` bash
-python -m pytest -sv ./tests/
-python -m pytest -sv ./examples/
-```
-
-By default, slow tests are skipped. Set the `RUN_SLOW` environment variable to `yes` to run them.
+Refer to the [contributing guide](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#tests) for details about running tests.
 
 ## OpenAI GPT original tokenization workflow