Add project introduction to maubot's defaults file

roles/matrix-bot-maubot/defaults/main.yml

@@ -1,5 +1,8 @@
 ---
 
+# maubot is a plugin-based Matrix bot system.
+# Project source code URL: https://mau.dev/maubot/maubot
+
 matrix_bot_maubot_enabled: true
 matrix_bot_maubot_container_image_self_build: false
 matrix_bot_maubot_docker_repo: "https://mau.dev/maubot/maubot.git"