Add a HOWTO on how to port from Python 2 to Python 3.
diff --git a/Doc/howto/index.rst b/Doc/howto/index.rst
index 09bc5cb..94ecc9a 100644
--- a/Doc/howto/index.rst
+++ b/Doc/howto/index.rst
@@ -14,6 +14,7 @@
    :maxdepth: 1
 
    advocacy.rst
+   pyporting.rst
    cporting.rst
    curses.rst
    descriptor.rst