"Echo" example from README should following RFC6121

[adefirmanf]

Based on RFC6121, attributes for sending message from README and example doesn't following the documentation. It produce the message doesn't receive to client.

I would suggest to PR the example for update README and example library. Adding "chat" as type will resolve this issue.

stanza.Message{Attrs: stanza.Attrs{To: msg.From, Type: "chat"}, Body: msg.Body}

Reference : https://xmpp.org/rfcs/rfc6121.html#message-syntax