Some docs for virtualenv support

docs/source/virtualenv.rst

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+`virtualenv` - Python virtual environments
+==========================================
+
+Synopsis::
+
+  fpm -s virtualenv [other options] EGG_SPEC|requirements.txt
+
+FPM has support for building packages that provide a python virtualenv from a
+single egg or from a `requirements.txt` file.  This lets you bundle up a set of
+python dependencies separate from system python that you can then distribute.
+
+.. note::
+   `virtualenv` support requires that you have `virtualenv` and  the
+   `virtualenv-tools` binary on your path.  This can usually be achieved with
+   `pip install virtualenv virtualenv-tools`.
+
+Example uses:
+=============
+
+Build an rpm package for ansible::
+
+  fpm -s virtualenv -t deb ansible
+  yum install virtualenv-ansible*.rpm
+  which ansible # /usr/share/python/ansible/bin/ansible
+
+Create a debian package for your project's python dependencies under `/opt`::
+
+  echo 'glade' >> requirements.txt
+  echo 'paramiko' >> requirements.txt
+  echo 'SQLAlchemy' >> requirements.txt
+  fpm -s virtualenv -t deb --name myapp-python-libs \
+    --virtualenv-install-location /opt/myapp \
+    requirements.txt
+
+Create a debian package from a version 0.9 of an egg kept in your internal
+pypi repository, along with it's external dependencies::
+
+  fpm -s virtualenv -t deb \
+    --virtualenv-pypi-extra-url=https://office-pypi.lan/ \
+    proprietary-magic=0.9