Document 'folder' attribute (#86)

#38 added the documentation for grafana_folder
but didn't add the extra attribute to grafana_dashboard

Author: jeohist

website/docs/r/dashboard.html.md

@@ -37,10 +37,12 @@ meta-parameter:
 The following arguments are supported:
 
 * `config_json` - (Required) The JSON configuration for the dashboard.
+* `folder` - (Optional, Forces new resource) The internal id of the folder
+  in Grafana in which to place the dashboard. 
 
 ## Attributes Reference
 
-The resource exports the following attributes:
+In addition to all arguments above, the following attributes are exported:
 
 * `slug` - A URL "slug" for this dashboard, generated by Grafana by removing
   certain characters from the dashboard name given as part of the `config_json`