Add documentation for directory "vmc" with pointers to existing web pages.

git-svn-id: http://root.cern.ch/svn/root/trunk@17867 27541ba8-7e3a-0410-8455-c3a389f83636

vmc/doc/index.txt

+This directory contains the VirtuaL MONTE CARLO classes.
+BEGIN_HTML
+ Several documents describing these classes are listed below:
+<ul>
+<li><a href="http://www.slac.stanford.edu/econf/C0303241/proc/papers/THJT006.PDF">The Virtual Monte Carlo paper
+      </A></li>
+<li><a href="http://root.cern.ch/root/vmc/VirtualMC.html">the Virtual Monte Carlo Web page
+    </a></li>
+<li><a href="http://root.cern.ch/root/vmc/Geant4VMC.html">the Geant4 Virtual Monte Carlo
+    </a></li>
+<li><a href="http://fca.web.cern.ch/fca/vmc.htm">Alice Virtual Monte Carlo
+      </A></li>
+</ul>
+END_HTML