docs: add credentials option for cookie-based sticky sessions

This is needed for cookie-based sticky sessions, for example with AWS
load balancers and the AWSALBCORS cookie.

Please note that the cookies must be created with `SameSite=None` and
`Secure`.

Reference: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Access-Control-Allow-Credentials

README.md

@@ -37,7 +37,8 @@ const httpServer = createServer();
 
 const io = new Server(httpServer, {
   cors: {
-    origin: ["https://admin.socket.io"]
+    origin: ["https://admin.socket.io"],
+    credentials: true // needed for cookie-based sticky sessions, else you can ignore this line
   }
 });