add info about how to build HTML documentation

svn path=/trunk/matplotlib/; revision=6481

Author: astraw

doc/README.txt

@@ -32,3 +32,9 @@ python documentation system built on top of ReST.  This directory contains
 
 * mpl_examples - a link to the matplotlib examples in case any
   documentation wants to literal include them
+
+To build the HTML documentation, install sphinx (0.5 or greater
+required), then type "python make.py html" in this directory.  Wait
+for the initial run (which builds the example gallery) to be done,
+then run "python make.py html" again. The top file of the results will
+be ./build/html/index.html