add demo readme

Author: dmackdev

demo/README.md

@@ -0,0 +1,21 @@
+# egui_json_tree demo app
+
+This demo demonstrates `egui_json_tree` and its capabilities in an `egui`/`eframe` app.
+
+See various use cases implemented in the [/apps](https://github.com/dmackdev/egui_json_tree/tree/main/demo/src/apps) folder, which includes:
+
+- Automatic expansion of arrays/objects and highlighting, based on search term matches.
+- Copying JSON paths and values to the clipboard.
+- A JSON editor UI.
+
+View this demo online at <https://dmackdev.github.io/egui_json_tree>.
+
+Run it locally via one of the commands below:
+
+```bash
+# Native target:
+cargo run -p demo --release
+
+# WASM/web target:
+trunk serve --release --open
+```