note demo in README.md

Author: peteroupc

README.md

@@ -30,6 +30,7 @@ platonic solids.  Demonstrates how vertex and index arrays are built up to creat
 a rotating, pulsating box.
 * [surfaces.html](https://peteroupc.github.io/html3dutil/surfaces.html) - Demonstrates
 using evaluators to generate parametric surfaces.
+* [stl.html](https://peteroupc.github.io/html3dutil/stl.html) - Demonstrates loading 3D models.
 
 Examples
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