add a small example

examples/client.rs

@@ -0,0 +1,11 @@
+extern crate xmpp;
+
+use xmpp::jid::Jid;
+use xmpp::client::ClientBuilder;
+
+use std::env;
+
+fn main() {
+    let jid: Jid = env::var("JID").unwrap().parse().unwrap();
+    let client = ClientBuilder::new(jid).connect().unwrap();
+}