Update README.md with support portal link

Author: aonemd

README.md

@@ -598,6 +598,7 @@ final socketId = await pusher.getSocketId();
 - If you have found a bug, please open an issue.
 - If you have a feature request, please open an issue.
 - If you want to contribute, please submit a pull request.
+- If you do not receive a timely response, feel free to check our [support portal](https://docs.bird.com/pusher).
 
 ## Credits