Makefile: Refer to CONTRIBUTING doc, reword test to avoid "example" (#5910)

CONTRIBUTING: Add concrete example for running single test

Author: EricCousineau-TRI

.github/CONTRIBUTING.md

@@ -108,7 +108,7 @@ Most of the tests in PyTorch Lightning train a trial MNIST model under various t
 To build the documentation locally, simply execute the following commands from project root (only for Unix):
 - `make clean` cleans repo from temp/generated files
 - `make docs` builds documentation under _docs/build/html_
-- `make test` runs all project's tests
+- `make test` runs all project's tests with coverage
 
 ### Original code
 
@@ -205,6 +205,12 @@ This is useful if you do not test against all required dependency versions.
 python -m pytest pytorch_lightning tests pl_examples -v
 ```
 
+You can also run a single test as follows:
+
+```bash
+python -m pytest -v tests/trainer/test_trainer_cli.py::test_default_args
+```
+
 ### Pull Request
 
 We welcome any useful contribution! For your convenience here's a recommended workflow:

Makefile

@@ -19,10 +19,11 @@ clean:
 	rm -rf ./docs/source/api
 
 test: clean
+	# Review the CONTRIBUTING docmentation for other ways to test.
 	pip install -r requirements/devel.txt
 	# install APEX, see https://github.com/NVIDIA/apex#linux
 
-	# use this to run tests
+	# run tests with coverage
 	python -m coverage run --source pytorch_lightning -m pytest pytorch_lightning tests pl_examples -v
 	python -m coverage report