Revert of Removed virtualenv from depot_tools

bootstrap/virtualenv/CONTRIBUTING.rst

Index: bootstrap/virtualenv/CONTRIBUTING.rst
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+virtualenv
+==========
+
+See docs/index.rst for user documentation.
+
+Contributor notes
+-----------------
+
+* virtualenv is designed to work on python 2 and 3 with a single code base.
+  Use Python 3 print-function syntax, and always ``use sys.exc_info()[1]``
+  inside the ``except`` block to get at exception objects.
+
+* virtualenv uses git-flow_ to `coordinate development`_. The latest stable
+  version should exist on the *master* branch, and new work should be
+  integrated to *develop*.
+
+* All changes to files inside virtualenv_embedded should be integrated to
+  ``virtualenv.py`` with ``bin/rebuild-script.py``.
+
+.. _git-flow: https://github.com/nvie/gitflow
+.. _coordinate development: http://nvie.com/posts/a-successful-git-branching-model/