Remove obsolete "extension", "description file" and "server" section

Documentation has been moved on Slicer wiki:
 http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/FAQ/Extensions#What_is_an_extension_.3F
 http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Extensions/Server
 http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Extensions/DescriptionFile
Merge branch 'update-Readme'

* update-Readme:
  Remove obsolete "extension", "description file" and "server" section