Documentation Request about Ping Pong

Hello nhooyr,

In gorilla/websocket, end-users must explicitly handle pings and pongs. From what I am reading on the issues forum, nhooyr/websocket handles this automatically. This is also implied by reading the source code from the [chat example](https://github.com/nhooyr/websocket/tree/master/examples/chat), which doesn't seem to have any ping pong code.

Can this please be documented front and center in the README? Right now, the README simply states that there is a "Ping pong API", but this doesn't communicate to the end user whether or not they should be writing pong pong code in their application or not.

Maybe the text of "Ping pong API" should be changed to "Automatic handling of pings and pongs, with an API exposing functionality for advanced users", or something along those lines. I would write a PR, but it's a one line change and you can probably word it the way you want yourself.

The reason that this seems important is that I expect typical writers of Go software will have used gorilla/websocket in the past, and are used to handling ping/pongs, so this will be a repeated point of confusion as more and more users use this library.

In closing, thanks for the library.