Revert of Removed virtualenv from depot_tools

bootstrap/virtualenv/docs/development.rst

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+Development
+===========
+
+Contributing
+------------
+
+Refer to the `pip development`_ documentation - it applies equally to
+virtualenv, except that virtualenv issues should filed on the `virtualenv
+repo`_ at GitHub.
+
+Virtualenv's release schedule is tied to pip's -- each time there's a new pip
+release, there will be a new virtualenv release that bundles the new version of
+pip.
+
+Files in the `virtualenv_embedded/` subdirectory are embedded into
+`virtualenv.py` itself as base64-encoded strings (in order to support
+single-file use of `virtualenv.py` without installing it). If your patch
+changes any file in `virtualenv_embedded/`, run `bin/rebuild-script.py` to
+update the embedded version of that file in `virtualenv.py`; commit that and
+submit it as part of your patch / pull request.
+
+.. _pip development: http://www.pip-installer.org/en/latest/development.html
+.. _virtualenv repo: https://github.com/pypa/virtualenv/
+
+Running the tests
+-----------------
+
+Virtualenv's test suite is small and not yet at all comprehensive, but we aim
+to grow it.
+
+The easy way to run tests (handles test dependencies automatically)::
+
+    $ python setup.py test
+
+If you want to run only a selection of the tests, you'll need to run them
+directly with pytest instead. Create a virtualenv, and install required
+packages::
+
+    $ pip install pytest mock
+
+Run pytest::
+
+    $ pytest
+
+Or select just a single test file to run::
+
+    $ pytest tests/test_virtualenv
+
+Status and License
+------------------
+
+``virtualenv`` is a successor to `workingenv
+<http://cheeseshop.python.org/pypi/workingenv.py>`_, and an extension
+of `virtual-python
+<http://peak.telecommunity.com/DevCenter/EasyInstall#creating-a-virtual-python>`_.
+
+It was written by Ian Bicking, sponsored by the `Open Planning
+Project <http://openplans.org>`_ and is now maintained by a
+`group of developers <https://github.com/pypa/virtualenv/raw/master/AUTHORS.txt>`_.
+It is licensed under an
+`MIT-style permissive license <https://github.com/pypa/virtualenv/raw/master/LICENSE.txt>`_.