eframe README: explain how to enable copy/paste

crates/eframe/README.md

@@ -28,6 +28,7 @@ You need to either use `edition = "2021"`, or set `resolver = "2"` in the `[work
 
 You can opt-in to the using [`egui_wgpu`](https://github.com/emilk/egui/tree/master/crates/egui_wgpu) for rendering by enabling the `wgpu` feature and setting `NativeOptions::renderer` to `Renderer::Wgpu`.
 
+To get copy-paste working on web, you need to compile with `export RUSTFLAGS=--cfg=web_sys_unstable_apis`.
 
 ## Alternatives
 `eframe` is not the only way to write an app using `egui`! You can also try [`egui-miniquad`](https://github.com/not-fl3/egui-miniquad), [`bevy_egui`](https://github.com/mvlabat/bevy_egui), [`egui_sdl2_gl`](https://github.com/ArjunNair/egui_sdl2_gl), and others.