docs(auth): add with-iron-session to the alternatives list (#11903)

Following https://github.com/zeit/next.js/pull/11865#issuecomment-613663718

examples/with-cookie-auth/README.md

@@ -3,3 +3,4 @@ This example has been deprecated and removed in favor of one of the following ex
 - [auth0](https://github.com/zeit/next.js/tree/canary/examples/auth0)
 - [with-cookie-auth-fauna](https://github.com/zeit/next.js/tree/canary/examples/with-cookie-auth-fauna)
 - [with-passport](https://github.com/zeit/next.js/tree/canary/examples/with-passport)
+- [with-iron-session](https://github.com/zeit/next.js/tree/canary/examples/with-iron-session)