Create README.md

Author: TimGoTheCreator

examples/GalaxyDemo/README.md

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+### Rotating Disk Galaxy Demo
+
+This example generates a **rotating disk galaxy** with 20 000 particles.  
+The disk has a radius of `0.5`, a vertical thickness of `0.2`, and a total mass of `1.0`.
+
+Disk setups like this are useful for testing **galaxy stability**, **rotation curves**, and **flattened mass distributions**.
+
+### Running this example from this directory
+
+```bash
+python galaxy.py
+../../next galaxy.txt 0.01 5
+```