Add a sentence to CONTRIBUTING how to run tests in parallel

[hagenw]

By using the pytest-xdist package one can execute tests in parallel, e.g.

$ python -m pytest -n 4

reduces the test execution time from 26.26s to 17.42s for me.

The only problem is that pytest-xdist does not work with every test definition.
When adding it to CONTRIBUTING, we should also add a test for running it in CI, looking at https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources there seems to be the option to use at least 2 workers.