Leave a note regarding coccinelle and vgz

in a place more likely to be noticed

I also played with other spatch options to replace ``--dir .``, but they
resulted in more clumsy shell commands.

Ref a98c1489

tools/coccinelle/README.rst

@@ -18,4 +18,7 @@ Unless noted otherwise, all patches should work when invoked as::
 The ``archive/`` directory contains patches which we used once and
 should not need again, but want to retain for reference.
 
+Do not commit any ``libvgz`` changes, as this code is manually kept in
+sync with upstream.
+
 .. _coccinelle: http://coccinelle.lip6.fr/