Short note on how to run nox tests

Author: ehossack

CONTRIBUTING.md

@@ -23,6 +23,8 @@ You'll need to have [nox](https://github.com/theacodes/nox) installed:
 
 * `pip install nox`
 
+You'll need to have all the available python versions installed.
+An example of running for just one version is `nox --extra-python=$(which python) -s test`
 With `nox`, you can run different sessions (default are `lint` and `test`):
 
 * `format` -> Format the code.