Add Python porting howto from 3.x.
No change was necessary to make the file build, contrary to cporting.rst
that uses markup like :c:func: that needs to be changed to :cfunc: in
2.7. Future backports should be straightforward.
diff --git a/Doc/howto/cporting.rst b/Doc/howto/cporting.rst
index 8030f13..46482ab 100644
--- a/Doc/howto/cporting.rst
+++ b/Doc/howto/cporting.rst
@@ -1,5 +1,7 @@
.. highlightlang:: c
+.. _cporting-howto:
+
********************************
Porting Extension Modules to 3.0
********************************