Make instances have a lib/python directory with a proper README.txt by default.

Note that it's already automatically put on the PYTHONPATH by Zope, it just
wasn't there yet by default.

skel/README.txt

@@ -5,5 +5,6 @@ server.  It contains the following directories:
   etc/         Configuration files
   Extensions/  Python sources for External Methods
   log/         Log files
-  Products/    Installed products specific to the instance
+  lib/python/  Installed Python packages specific to the instance
+  Products/    Installed Zope Products specific to the instance
   var/         Run-time data files, including the object database

skel/lib/python/README.txt.in

+This directory will hold any additional Python packages installed into
+this Zope instance.  Installing Distutils-based packages should be
+done using the command:
+
+  <<PYTHON>> setup.py install --home <<INSTANCE_HOME>>
+
+This will cause the newly installed Python packages to appear in this
+directory.