Ignore Chef config, provide example file

This allows admins to adjust the config to their needs, e.g. using an
absolute path for `encrypted_data_bag_secret`.

.gitignore

@@ -1,5 +1,6 @@
 /.chef/encrypted_data_bag_secret
-.chef/chef_guid
+/.chef/chef_guid
+/.chef/config.rb
 /.bundle/
 /.vagrant/
 /.kitchen