Update hello world example and add config example

examples/helloworld/helloworld.py

@@ -0,0 +1,10 @@
+from mautrix.types import EventType
+from maubot import Plugin, MessageEvent
+from maubot.handlers import event
+
+
+class HelloWorldBot(Plugin):
+    @event.on(EventType.ROOM_MESSAGE)
+    async def handler(self, event: MessageEvent) -> None:
+        if event.sender != self.client.mxid:
+            await event.reply("Hello, World!")