Readme note.

README.md

@@ -16,6 +16,8 @@ Transitions are reset when the slideshow wraps around.
 
 You'll need the [Rust toolchain](https://rustup.rs/).
 
+Use [this demo](https://www.egui.rs/index.html) to better understand the [egui](https://github.com/emilk/egui) library.
+
 General compilation instructions can be found [here](https://github.com/emilk/eframe_template/blob/master/README.md).
 
 ### Linux