Add documentation

website/docs/r/app_service.html.markdown

@@ -189,6 +189,8 @@ The following arguments are supported:
 
 * `websockets_enabled` - (Optional) Should WebSockets be enabled?
 
+* `cors` - (Optional) CORS settings
+
 ---
 
 `ip_restriction` supports the following:
@@ -241,6 +243,12 @@ The following attributes are exported:
 * `repo_url` - URL of the Git repository for this App Service.
 * `branch` - Branch name of the Git repository for this App Service.
 
+---
+
+`cors` supports the following:
+
+* `allowed_origins` - (Required) List of allowed origins
+
 ## Import
 
 App Services can be imported using the `resource id`, e.g.