Add a README to use varnish recipe.

slapos/recipe/README.generic_varnish.txt

+generic_varnish
+===============
+
+This recipe creates a varnish instance dedicated for ERP5 with a web checker[1]
+set up.
+
+How to Use
+==========
+
+On slap console, you can instanciate varnish like this:
+
+instance = request(
+  software_type='varnish',
+  partition_parameter_kw={
+     'tidstorage-url':'http://[your tidstrage address]:your tid strage port',
+     'web-checker-mail-address':'web-checker-result@example.com',
+     'web-checker-smtp-host':'mail.example.com',
+  }
+)
+
+tidstrage-url is the backend url that varnish will cache. It is expected that
+the backend is created by tidstorage recipe.
+
+web-checker-mail-address is the email address where web checker will send
+the HTTP Cache cheking result.
+
+web-checker-smtp-host is the smtp server to be used to send the web checker
+result.
+
+TODO
+====
+
+We need to merge this and apache_frontend recipe.
+
+References
+==========
+
+[1] web_checker (it is a part of erp5.util)
+http://pypi.python.org/pypi/erp5.util
+web_checker: Web site HTTP Cache header checking tool
+