[egui_web] Document required RUSTFLAGS=--cfg=web_sys_unstable_apis
This crates allows you to compile GUI code written with [Egui](https://crates.io/crates/egui) to [WASM](https://en.wikipedia.org/wiki/WebAssembly) to run on a web page.
Check out [docs/index.html](https://github.com/emilk/egui/blob/master/docs/index.html), [demo_web](https://github.com/emilk/egui/tree/master/demo_web) and [build_web.sh](https://github.com/emilk/egui/blob/master/build_web.sh) for examples of how to set it up.
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+To use `egui_web`, you need to set the `RUSTFLAGS=--cfg=web_sys_unstable_apis` flag.
