Add descriptor HOWTO to py3k docs.
diff --git a/Doc/howto/index.rst b/Doc/howto/index.rst
index 7d64688..0335a80 100644
--- a/Doc/howto/index.rst
+++ b/Doc/howto/index.rst
@@ -16,6 +16,7 @@
    advocacy.rst
    cporting.rst
    curses.rst
+   descriptor.rst
    doanddont.rst
    functional.rst
    regex.rst