Adapt docstring for the zip_gallery plugin as suggested by @saimn

sigal/plugins/zip_gallery.py

@@ -22,6 +22,10 @@
 
 To ignore a ZIP gallery generation for a particular album, put a ``.nozip_gallery`` file next to it in its parent
 folder. Only the existence of this ``.nozip_gallery`` file is tested.
+If no .nozip_fallery file is present, then make a ZIP archive with all media files.
+
+If the ``zip_gallery`` setting is set,it contains the location of a zip
+archive with all original images of the corresponding directory.
 """
 
 import logging