[Orgmode] Attention: Change of default file name for publishing sitemap

[Carsten Dominik]

Hi,

org-publish is able to create a sitemap of a project.  The default  
file name
for this "index-file" was index.org, a terrible default since it  
overwrites
the main page of a site.

The new default if sitemap.org - please verify that your projects will
work after the change.

- Carsten