add the beginnings of a C-API 2 -> 3 porting guide
diff --git a/Doc/howto/index.rst b/Doc/howto/index.rst
index 5a1f397..7d64688 100644
--- a/Doc/howto/index.rst
+++ b/Doc/howto/index.rst
@@ -14,6 +14,7 @@
    :maxdepth: 1
 
    advocacy.rst
+   cporting.rst
    curses.rst
    doanddont.rst
    functional.rst