FOF updates: removal of update server and addition of a README.txt file

This pull requests addresses issues [discussed on the mailing list](https://groups.google.com/forum/#!topic/joomla-dev-cms/lA4CLwi1ZDE), namely:

* Removing the update server; FOF will only be updated when Joomla! itself is updated
* Addition of a README.txt file explaining what FOF is and giving links to its resources