Added ged/index.txt for GED.

git-svn-id: http://root.cern.ch/svn/root/trunk@21426 27541ba8-7e3a-0410-8455-c3a389f83636

ged/doc/index.txt

+This directory contains the Graphics Editor (GED) of ROOT and the basic classes of so-called object editors.
+BEGIN_HTML
+ See the chapter about the Graphics Editor in ROOT on page 418 at:
+<ul>
+<li><a href="ftp://root.cern.ch/root/doc/25WritingGUI.pdf">The Chapter about Writing a Graphical User Interface in the Users Guide
+    </a></li>
+<li>The different object editors are documented in the corresponding classes.</li>
+</ul>
+END_HTML