add dox ref

Author: trbabb

README.md

@@ -13,6 +13,10 @@ entttree::HierarchySystem<RenderH>    scene(reg);
 entttree::HierarchySystem<CollisionH> collision(reg);
 ```
 
+## Documentation
+
+[API Reference](https://tbabb.github.io/entttree/)
+
 ## Systems
 
 ### HierarchySystem