further update README for test running changes

Author: ahupp

README.md

@@ -101,22 +101,23 @@ triage it.
 
 ## Running the tests
 
-To run the tests across a variety of linux distributions (depends on Docker):
+We use the `tox` test runner which can be installed with `python -m pip install tox`.
+
+To run tests locally across all available python versions:
 
 ```
-./test_docker.sh
+python -m tox
 ```
 
-To run tests locally across all available python versions:
+Or to run just against a single version:
 
 ```
-./test/run.py
+python -m tox py
 ```
-
-To run against a specific python version:
+To run the tests across a variety of linux distributions (depends on Docker):
 
 ```
-LC_ALL=en_US.UTF-8 python3 test/python_magic_test.py
+./test/run_all_docker_test.sh
 ```
 
 ## libmagic python API compatibility