Add information about SourceForge

Author: mdboom

downloads.html

@@ -41,6 +41,9 @@ <h1>Downloads</h1>
             directly, they are available
             on <a href="http://pypi.python.org/pypi/matplotlib">PyPI</a>.</p>
 
+            <p>Older releases, prior to version 1.2.0, are available
+            on <a href="http://sourceforge.net/projects/matplotlib/files/">SourceForge</a>.
+
             <p>As of matplotlib 1.5, we are no longer making downloads
             available on SourceForge.</p>