update docs on using virtualenv

doc/INSTALL.rst

@@ -48,7 +48,10 @@ Zope can be installed using ``easy_install`` either using a global
 easy_install installation or within a virtualized Python environment
 (using ``virtualenv``)::
 
-  $ /path/to/easy_install -i http://download.zope.org/Zope2/index/<Zope version> Zope2
+  $ virtualenv --no-site-packages my_zope
+  $ cd my_zope
+  $ source bin/my_zope
+  $ bin/easy_install -i http://download.zope.org/Zope2/index/<Zope version> Zope2
 
 This will create the related scripts like ``mkzopeinstance`` within the
 ``bin`` folder of you global or virtualized Python environment.