more doc for rerun itself

examples/cu_pointclouds/README.md

@@ -1,3 +1,10 @@
 # cu_pointclouds
 
 This demonstrates how to use Copper to interface with rerun to visualize its standard pointcloud. 
+
+To use it:
+```
+$ cargo install rerun-cli
+$ rerun
+$ cargo run   # it will automagically show the point cloud in rerun with the default parameters.
+```