Add section describing how to run tests

Author: kotfu

README.md

@@ -9,3 +9,42 @@ $ pip install cmd2-submenu
 
 ## How to use
 
+## Running tests
+
+`cmd2` uses a three tiered testing strategy to test on multiple versions
+of python on various platforms. This plugin uses the same strategy:
+
+- [pytest](https://pytest.org) runs the unit tests
+- [tox](https://tox.readthedocs.io/) runs the unit tests on multiple versions
+  of python
+- [AppVeyor](https://www.appveyor.com/) and [TravisCI](https://travis-ci.com)
+  run the tests on the various supported platforms
+
+### Running unit tests
+
+Run `pytest` from the top level directory of this plugin to run all the
+unit tests.
+
+### Use tox to run unit tests in multiple versions of python
+
+The included `tox.ini` is setup to run the unit tests in python 3.4, 3.5,
+and 3.6. In order for `tox` to work, you need to have different versions of
+python executables available in your path.
+[pyenv](https://github.com/pyenv/pyenv) is one method of doing this easily.
+Once `pyenv` is installed, use it to install multiple versions of python:
+
+```
+$ pyenv install 3.4.8
+$ pyenv install 3.5.5
+$ pyenv install 3.6.5
+$ pyenv local 3.6.5 3.5.5 3.4.8
+```
+
+This will create a `.python-version` file and instruct the `pyenv` shims
+to make `python3.6`, `python3.5`, and `python3.4` launch the appropriate
+versions of python.
+
+Once these executables are configured, invoking `tox` will create a
+virtual environment for each version of python, install the prerequisite
+packages, and run your unit tests.
+