add messaging demonstration to example

example/message.dart

@@ -0,0 +1,39 @@
+import 'package:whixp/whixp.dart';
+
+void main() {
+  final whixp = Whixp(
+    jabberID: 'vsevex@localhost',
+    password: 'passwd',
+    logger: Log(enableWarning: true, enableError: true, includeTimestamp: true),
+    internalDatabasePath: 'whixp',
+    reconnectionPolicy: RandomBackoffReconnectionPolicy(1, 3),
+  );
+
+  whixp
+    ..addEventHandler('streamNegotiated', (_) {
+      whixp.sendPresence();
+
+      /// Sending sample message.
+      whixp.sendMessage(JabberID('alyosha@localhost'), body: 'First message');
+
+      /// Sending marked message.
+      const id = StanzaID('random-id');
+      whixp.sendMessage(
+        JabberID('alyosha@localhost'),
+        body: 'Second message requesting displayed marker',
+        requestDisplayedInformation: true,
+        payloads: [id],
+      );
+    })
+    ..addEventHandler<Message>('message', (message) {
+      if (message == null) return;
+      if (message.isMarked) {
+        final messageIDs = message.get<StanzaID>();
+        if (messageIDs.isNotEmpty && message.from != null) {
+          final id = messageIDs.first.id;
+          whixp.sendDisplayedMessage(message.from!, messageID: id);
+        }
+      }
+    });
+  whixp.connect();
+}