update testing description

Author: josesho

dabest/tests/README.md

@@ -1,9 +1,8 @@
 # Testing
 
-We use [pytest](https://docs.pytest.org/en/latest) to execute the tests. More documentation of the testing paradigm will be added in the near future.
+We use [pytest](https://docs.pytest.org/en/latest) to execute the tests. For testing of plot generation, we use the [mpl plugin](https://github.com/matplotlib/pytest-mpl) for pytest. A range of different plots are created, and compared against the baseline images in the `baseline_images` subfolder.
 
 To run the tests, go to the root of this repo directory and run 
-
 ```shell
 pytest dabest
 ```