From a post on http://diveintohtml5.org/offline.html (Wayback Machine Copy)
Your cache manifest file can be located anywhere on your web server, but it must be served with the content type text/cache-manifest. If you are running an Apache-based web server, you can probably just put an AddType directive in the .htaccess file at the root of your web directory:
AddType text/cache-manifest .manifest
Where do I create the file .htaccess?
Need some more setup with apachectl ?
Thanks very much !