Add tests README

test/README.md

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+# Docker stacks testing
+
+We test our images using `pytest` module.
+
+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
+- Image specific tests - for example, [base-notebook/test](https://github.com/jupyter/docker-stacks/blob/master/base-notebook/test) folder
+
+We also have a way to easily run arbitrary python files in a container.
+This is useful for running unit tests of packages we use, so we put these files in `{image}/test/units` folder.
+An example of such a test is [unit_pandas.py](https://github.com/jupyter/docker-stacks/blob/master/scipy-notebook/test/units)