add note to expires option

Author: jhuleatt

README.md

@@ -86,6 +86,8 @@ build log.
 
 ### `expires` _{string}_
 
+> ⚠️ This option isn't supported yet. If you set it, it will be ignored. See [issue #7](https://github.com/FirebaseExtended/action-hosting-deploy/issues/7) for context.
+
 The length of time the channel should live. Default is 7 days.
 
 ### `projectId` _{string}_