Clarify current state of HTTP/2 support

[ptone]

No description provided.

[ahmetb]

can you run npx markdown-toc -i README.md to update ToC?

[ahmetb]

I think "will not route" should be "will downgrade requests to HTTP/1".

H2C is designed to be compatible with HTTP1.

[ptone]

You can write a HTTP/2 only without downgrade, and require prior-knowledge - but this is in the weeds maybe:

// This server only supports "H2C prior knowledge".
// You can add standard HTTP/2 support by adding a TLS config.
func H2CServerPrior() {
	server := http2.Server{}

	l, err := net.Listen("tcp", "0.0.0.0:8080")
	checkErr(err, "while listening")

	fmt.Printf("Listening [0.0.0.0:8080]...\n")
	for {
		conn, err := l.Accept()
		checkErr(err, "during accept")

		server.ServeConn(conn, &http2.ServeConnOpts{
			Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
				fmt.Printf("New Connection: %+v\n", r)
				fmt.Fprintf(w, "Hello, %v, http: %v", r.URL.Path, r.TLS == nil)
			}),
		})
	}
}