Update readme with instructions for running tests locally

Author: ashimaathri

README.rst

@@ -26,3 +26,15 @@ necessary pieces for building a SAML2 service provider or an identity provider.
 The distribution contains examples of both.
 Originally written to work in a WSGI environment there are extensions that
 allow you to use it with other frameworks.
+
+Testing
+=======
+PySAML2 uses the `pytest <http://doc.pytest.org/en/latest/>`_ framework for
+testing. To run the tests on your system's version of python 
+
+1. Create and activate a `virtualenv <https://virtualenv.pypa.io/en/stable/>`_.
+2. Inside the virtualenv, install the dependencies needed for testing :code:`pip install -r tests/test_requirements.txt`
+3. Run the tests :code:`py.test tests`
+
+To run tests in multiple python environments, you can use
+`pyenv <https://github.com/yyuu/pyenv>`_ with `tox <https://tox.readthedocs.io/en/latest/>`_.