Extensions (such as the hieroglyphic module WikiHiero) are distributed
separately. Drop them into this extensions directory and enable as
per the extension's directions.

If you are a developer, you want to fetch the extension tree in another
directory and make a symbolic link:

 mediawiki/extensions$ ln -s ../../extensions-trunk/FooBarExt

The extensions are available through svn at:
    http://svn.wikimedia.org/svnroot/mediawiki/trunk/extensions/

You can find documentation and additional extensions on MediaWiki website:
    http://www.mediawiki.org/wiki/Category:Extensions