Add pytest info

test/README.md

@@ -2,6 +2,9 @@
 
 We test our images using `pytest` module.
 
+`conftest.py` and `pytest.ini` in the root of our repository define the environment in which tests are run.
+More info on pytest can be found [here](https://docs.pytest.org/en/latest/reference/index.html).
+
 There are two kinds of tests we use:
 
 - General tests - these are located in [this](https://github.com/jupyter/docker-stacks/blob/master/test) folder