Mention the servername option

Fixes #1

README.md

@@ -11,12 +11,15 @@ Returns an object with an `alpnProtocol` property. The `socket` property may be
 ```js
 const result = await resolveALPN({
 	host: 'nghttp2.org',
-	ALPNProtocols: ['h2', 'http/1.1']
+	ALPNProtocols: ['h2', 'http/1.1'],
+	servername: 'nghttp2.org'
 });
 
 console.log(result); // {alpnProtocol: 'h2'}
 ```
 
+**Note:** While the `servername` option is not required in this case, many other servers do. It's best practice to set it anyway.
+
 #### options
 
 Same as [TLS options](https://nodejs.org/api/tls.html#tls_tls_connect_options_callback).