Create README.md

Author: TimGoTheCreator

examples/ColdCollapseGalaxy/README.md

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+### Cold Collapse Demo
+This example is a plummer galaxy with all velocities set to 0, also used for stress tests. 
+
+To run this example, in this directory do
+
+```bash
+python coldcollapse.py
+../../next coldcollapse.txt 0.01 1
+```