Add the doc files for "geom"

git-svn-id: http://root.cern.ch/svn/root/trunk@17877 27541ba8-7e3a-0410-8455-c3a389f83636

geom/doc/index.txt

+This directory contains all the geometry  classes.
+BEGIN_HTML
+ Several documents describing these classes are listed below:
+<ul>
+<li>The main histogram class is documented in class TGeoManager.</li>
+<li><a href="ftp://root.cern.ch/root/doc/chapter19.pdf">The Chapter about the TGeo classes in the Users Guide
+    </a></li>
+</ul>
+END_HTML