docs: add README

libgdx/README.md

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+# libGDX-tutorial
+
+Learning materials and sample code for libGDX game development in Java.
+
+Multi-module Gradle project with android, desktop, and html targets.
+
+## Run
+
+```bash
+./gradlew desktop:run     # desktop launcher
+./gradlew html:superDev   # web (GWT)
+./gradlew android:installDebug  # android
+```
+
+## License
+
+Apache-2.0