Update server example to mention HTTP/2

As discussed in #89

Author: fluffysquirrels

examples/server.rs

@@ -2,6 +2,8 @@
 //!
 //! First parameter is the mandatory port to use.
 //! Certificate and private key are hardcoded to sample files.
+//! hyper will automatically use HTTP/2 if a client starts talking HTTP/2,
+//! otherwise HTTP/1.1 will be used.
 use core::task::{Context, Poll};
 use futures_util::{
     stream::{Stream, StreamExt},