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

git-svn-id: http://root.cern.ch/svn/root/trunk@17873 27541ba8-7e3a-0410-8455-c3a389f83636

roofit/doc/index.txt

+This directory contains the RooFit toolkit classes.
+BEGIN_HTML
+ See:
+<ul>
+<li><a href="http://roofit.sourceforge.net">The RooFit Toolkit for Data Modeling
+    </a></li>
+</ul>
+END_HTML