Update README.md about obsolete

README.md

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 # kambi_lines
 
+## 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
+
 Clone of an old "Color lines" game. You need to arrange colored balls in lines, to make space on the board.
 
 Concept is *very* closely based on an old DOS "Color lines" game by Olga Demin (programming), Igor Ivkin and Gennady Denisov (graphics). Credit for graphics goes to Igor Ivkin and Gennady Denisov, we merely use their graphics.