[DOC][MINOR] Added step for dev env setup (mlflow#4555)

* [DOC][MINOR] Added step for dev env setup
Signed-off-by: Vilmos Agoston <vilmos.agoston@gmail.com>

* purpose of mlflow-test-plugin clarified
Signed-off-by: Vilmos Agoston <vilmos.agoston@gmail.com>

Co-authored-by: Vilmos Agoston <vilmos.agoston@gmail.com>

CONTRIBUTING.rst

@@ -149,6 +149,7 @@ If you plan on doing development and testing, you will also need to install the
     pip install -r dev-requirements.txt
     pip install -r test-requirements.txt
     pip install -e .[extras]  # installs mlflow from current checkout
+    pip install -e tests/resources/mlflow-test-plugin # installs `mlflow-test-plugin` that is required for running certain MLflow tests
 
 You may need to run ``conda install cmake`` for the test requirements to properly install, as ``onnx`` needs ``cmake``.