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

git-svn-id: http://root.cern.ch/svn/root/trunk@17861 27541ba8-7e3a-0410-8455-c3a389f83636

hist/doc/index.txt

+This directory contains all the basic histogram classes.
+BEGIN_HTML
+ Several documents describing the I/O classes are listed below:
+<ul>
+<li>The main histogram class is documented in class TH1.</li>
+<li><a href="http://root.cern.ch/root/HowtoHistogram.html">How to use the Histogram classes
+    </a></li>
+<li><a href="http://root.cern.ch/root/HowtoMerge.html">How to Merge files with histograms and Trees
+      </A></li>
+<li><a href="http://root.cern.ch/root/HowtoFit.html">How to Fit Histograms
+      </A></li>
+<li><a href="http://root.cern.ch/root/htmldoc/examples/peaks.C.html">How to Fit find peaks in histograms
+      </A></li>
+<li><a href="http://root.cern.ch/root/HowtoConvert.html">How to convert Hbook/PAW files to ROOT
+      </A></li>
+</ul>
+END_HTML