Update README.md about obsolete

README.md

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 # Darkest Before the Dawn
 
+## This is a very old application; do not use this to learn Castle Game Engine!
+
+This application was developed using a very old _Castle Game Engine_ version. It still builds with the latest engine version (it is even tested by [GitHub Actions](https://castle-engine.io/github_actions) to make sure we maintain backward-compatibility) but it's absolutely *not* how I would go about implementing this game type now.
+
+*Do not use this game as a learning example.* Instead, get [latest Castle Game Engine](https://castle-engine.io/) and browse `examples` inside the installed engine.
+
+This project is maintained here only:
+- for historical purposes
+- and as part of automated test to make sure we maintain backward-compatibility when developing new engine versions.
+
+## Introduction
+
 A scary 3D game. You're only safe within the light.
 
 Game information: https://castle-engine.io/darkest_before_dawn.php .