add some testing notes of the readme to the documentation

docs/docs/testing.rst

@@ -3,6 +3,22 @@
 Jedi Testing
 ============
 
+The test suite depends on ``tox`` and ``pytest``::
+
+    pip install tox pytest
+
+To run the tests for all supported Python versions::
+
+    tox
+
+If you want to test only a specific Python version (e.g. Python 2.7), it's as
+easy as::
+
+    tox -e py27
+
+Tests are also run automatically on `Travis CI
+<https://travis-ci.org/davidhalter/jedi/>`_.
+
 You want to add a test for |jedi|? Great! We love that. Normally you should
 write your tests as :ref:`Blackbox Tests <blackbox>`. Most tests would
 fit right in there.