Finished the "porting to 3.0" section.  Had to add a label to
howto/cporting.rst.
diff --git a/Doc/howto/cporting.rst b/Doc/howto/cporting.rst
index 3451f5c..3102fb5 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
 ********************************