Add note about running just tests

Author: blyxxyz

CONTRIBUTING.md

@@ -10,6 +10,8 @@ tox
 
 This will install and run all the tooling.
 
+`tox` aborts early if one of the steps fails. To run just the tests (for example if you can't get `mypy` to work), install and run [`pytest`](https://docs.pytest.org/en/latest/getting-started.html). To run just one particular test, run `pytest -k <name of test>`.
+
 ## Mypy
 
 [Mypy](https://mypy.readthedocs.io/en/stable/) is used for static typing. This is also managed by `tox`.