docs: add preview

Author: knowald

README.md

@@ -4,6 +4,8 @@
 
 Interactive tree visualizations for your Home Assistant entity hierarchy.
 
+![HA Hypertree preview](public/preview.png)
+
 Preview available at [hypertree.eightypercent.dev](https://hypertree.eightypercent.dev).
 
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