Added plugin repository file.

This file is read by Protege Desktop's autoupdate code.  Protege 4.3
reads this file from a Google Code repository
(http://protege.googlecode.com/svn/trunk/autoupdate/4_2/4.2-plugins-2012-03-19.repository).
For the 5.x series, Protege will read this file instead.  If plug-in
developers want their plugins added to this repository file, they should
submit pull requests.