Added docs for gotchas when developing on gentoo

HACKING.rst

@@ -140,6 +140,10 @@ M2Crypto installed using apt::
 
     virtualenv --system-site-packages /path/to/your/virtualenv
 
+On Gentoo systems you should use ``--system-site-packages`` when creating
+the virtualenv to enable pkg and portage_config functionality as the
+portage package is not available via pip
+
 .. note:: Using your system Python modules in the virtualenv
 
     If you have the required python modules installed on your system already

doc/topics/development/hacking.rst

@@ -35,9 +35,13 @@ Create a new `virtualenv`_:
 .. _`virtualenv`: https://pypi.python.org/pypi/virtualenv
 
 Avoid making your :ref:`virtualenv path too long <too_long_socket_path>`.
+
 On Arch Linux, where Python 3 is the default installation of Python, use
 the ``virtualenv2`` command instead of ``virtualenv``.
 
+On Gentoo you must use ``--system-site-packages`` to enable pkg and portage_config
+functionality
+
 .. note:: Using system Python modules in the virtualenv
 
     To use already-installed python modules in virtualenv (instead of having pip