Add testing docs to README

README.md

@@ -19,3 +19,19 @@ config = Bunch(access_key='a_key',
 provider = CloudProviderFactory().create_provider(ProviderList.EC2, config)
 print(provider.security.list_key_pairs())
 ```
+
+### Running tests
+To run the test suite locally, install [tox](https://tox.readthedocs.org/en/latest/)
+with `pip install tox` and run `tox` command. This will run all the tests for
+all the environments defined in file `tox.ini`. In order to properly run the
+tests, you should have all the environment variables listed in
+`tox.ini` file (under `passenv`) exported.
+
+If you’d like to run the tests on a specific environment only, use a command
+like this: `tox -e py27` (or ``python setup.py test`` directly). If you'd
+like to run the tests for a specific cloud only, you should export env var
+``CB_TEST_PROVIDER`` and specify the desired provider name (e.g., ``ec2`` or
+``openstack``) and then run the ``tox`` command.
+
+Note that running the tests will create various cloud resources, for which you
+may incur costs.