Update readme

README.md

@@ -7,5 +7,42 @@ No-std cross-platform Rust GameBoy emulator library. Rust GameboY (RGY, or Real
 [![License](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
 [![Actions Status](https://github.com/YushiOMOTE/rgy/workflows/Rust/badge.svg)](https://github.com/YushiOMOTE/rgy/actions)
 
-![demo](https://raw.github.com/wiki/YushiOMOTE/gbr/media/demo.gif)
-![screens](https://raw.github.com/wiki/YushiOMOTE/gbr/media/demo_screens.jpg)
+<img src="https://raw.github.com/wiki/YushiOMOTE/gbr/media/demo.gif" width="450" />
+<img src="https://raw.github.com/wiki/YushiOMOTE/gbr/media/demo_screens_2.jpg" width="450" />
+
+### Usage
+
+Once you implement OS-specific part, i.e. `Hardware` trait, you will get a GameBoy emulator for your environment.
+
+```rust
+struct Hardware;
+
+// 1. Implement `rgy::Hardware`.
+impl rgy::Hardware for Hardware {
+    ...
+}
+
+// 2. Call `rgy::run`.
+fn main() {
+    let cfg = Config::new();
+    let rom = include_bytes!("rom,gb");
+    rgy::run(cfg, &rom, Hardware);
+}
+```
+
+### Example
+
+```
+$ cargo run --example pc <a ROM file>
+```
+
+The example runs the GameBoy emulator in UNIX environment. It depends on `libasound2-dev` and `libxcursor-dev`.
+The ROM files can be easily downloaded from the Internet.
+
+### Projects
+
+The following projects use this library to run a GameBoy emulator.
+
+* [stickboy](https://github.com/yushiomote/stickboy) runs a GameBoy emulator on Macbook Pro (UEFI).
+* [biboy](https://github.com/yushiomote/biboy) runs a GameBoy emulator on BIOS PC.
+* [waboy](https://github.com/yushiomote/waboy) runs a GameBoy emulator on web browsers (wasm).