move varnish to docs; convert to reStructuredText

Author: buhman

docs/varnish.rst

@@ -0,0 +1,28 @@
+If specified, the ``VARNISH_BASE`` configuration option is used is
+used to invalidate varnish's cache when necessary. This should be the
+URL of the varnish instance; see `config.yaml.example
+<config.yaml.example>`_.
+
+This depends on a varnish configuration that processes ``BAN``
+requests over HTTP:
+
+.. code:: vcl
+
+    acl purge {
+        "localhost";
+    }
+
+    sub vcl_recv {
+        if (req.method == "BAN") {
+            if (!client.ip ~ purge) {
+                return(synth(405, "Not allowed."));
+            }
+
+            ban("req.http.host == " + req.http.host +
+            " && req.url ~ " + req.url);
+
+            return(synth(200, "Ban added."));
+        }
+    }
+
+Enjoy.