Add docs for class-based handlers

docs/dispatcher/class_based_handlers/message.md

@@ -0,0 +1,25 @@
+# MessageHandler
+
+There is base class for message handlers.
+
+## Simple usage:
+```pyhton3
+from aiogram.handlers import MessageHandler
+
+...
+
+@router.message_handler()
+class MyTestMessageHandler(MessageHandler):
+    filters = [Text(text="test")]
+
+    async def handle() -> Any:
+        return SendMessage(chat_id=self.chat.id, text="PASS")
+
+```
+
+## Extension
+
+This base handler is subclass of [BaseHandler](basics.md#basehandler) with some extensions:
+
+- `self.chat` is alias for `self.event.chat`
+- `self.from_user` is alias for `self.event.from_user`