Document an example of cookie-persistance using the WebClient

[wu-lee]

trafficstars

I mentioned this on the google group. Julien Viet confirmed that it does not support sessions, but since the WebClient is meant to be a rich client which makes these things easy, suggested I open an issue to request that some mechanism to support cookie persistence be added.

(Originally filed on the wrong project, but perhaps it makes sense to keep it since an example may be useful. Please feel free to delete.)

[wu-lee]

See vert-x3/vertx-web#648

[chengenzhao]

Yeah, we also hope the webclient could support cookies if it is simple and easy, please allow us to enable it