Document the optional dependencies for PNG output.

Hopefully this will help with #2

pages/home.rst

@@ -94,6 +94,9 @@ pygal uses `lxml <http://lxml.de/>`_ to generate the svg, this is the only neede
 Optional dependencies
 ---------------------
 
+PNG output requires `CairoSVG <http://cairosvg.org/>`_, `tinycss <http://packages.python.org/tinycss/>`_ and `cssselect <http://packages.python.org/cssselect/>`_.
+Install those with ``pip install CairoSVG tinycss cssselect``
+
 Unit testing needs `py.test <http://pytest.org/latest/>`_ or `nosetests <http://readthedocs.org/docs/nose/en/latest/>`_.
 
 Visual testing is based on `flask <http://flask.pocoo.org/>`_.