Add warning README.rst and fix pep8.

mediagoblin/plugins/piwigo/README.rst

@@ -0,0 +1,23 @@
+===================
+ piwigo api plugin
+===================
+
+.. danger::
+   This plugin does not work.
+   It might make your instance unstable or even insecure.
+   So do not use it, unless you want to help to develop it.
+
+.. warning::
+   You should not depend on this plugin in any way for now.
+   It might even go away without any notice.
+
+Okay, so if you still want to test this plugin,
+add the following to your mediagoblin_local.ini:
+
+.. code-block:: ini
+
+   [plugins]
+   [[mediagoblin.plugins.piwigo]]
+
+Then try to connect using some piwigo client.
+There should be some logging, that might help.