Update contributing.md

Author: LucaMarconato

docs/contributing.md

@@ -78,6 +78,14 @@ out to the developers of the dependency before the package is released to a wide
 
 [scanpy-test-docs]: https://scanpy.readthedocs.io/en/latest/dev/testing.html#writing-tests
 
+### Testing the correctness of the plots
+
+Many tests will produce plots and check that they are correct by comparing them with a previously saved and serialized version of the same plots. The ground truth images are located in `tests/_images`. Different OS/versions may produce similar but not identical plots (for instance the ticks/padding could vary). To take into account for this please consider the following:
+- you should not use locally generated plots as ground truth images, but you should commit images that have been generated by a GitHub Action. The recommended workflow is to go to the ["actions" page for the repo](https://github.com/scverse/spatialdata-plot/actions/workflows/test.yaml), download the artifacts, and upload them as ground truth (after having reviewed them).
+- the ground truth images need to be updated when a new test is passing, or when a test starts producing a slightly different (but consistent) plot.
+- please never replace the ground truth images without having manually reviewed them.
+- if you run the tests locally in macOS or Windows they will likely fail because the ground truth images are generated using Ubuntu. To overcome this you can use `act`, which will generate a Docker reproducing the environment used in the GitHub Action. After the Docker container is generated you can use it within IDEs to run tests and debug code.
+
 ## Publishing a release
 
 ### Updating the version number