Add new doc directory and v520 release notes

git-svn-id: http://root.cern.ch/svn/root/trunk@23589 27541ba8-7e3a-0410-8455-c3a389f83636

montecarlo/doc/v520.html

+<br>
+<hr>
+<h3>MonteCarlo</h3>
+The new <b>montecarlo</b> directory groups the packages 
+<ul>
+<li><b>eg</b>: Interface to event generators
+<li><b>g4root</b>: Interface to the Geant4 system
+<li><b>pythia6</b>: Interface to pythia6
+<li><b>pythia8</b>: Interface to pythia8
+<li><b>vmc</b>: The Virtual MonteCarlo
+</ul>