chore: update README.md

Author: zetanumbers

README.md

@@ -7,7 +7,32 @@ Idiomatic bindings for WASM-4 in Rust.
 cartridges (ROMs) are small, self-contained `.wasm` files that can be built with any programming
 language that compiles to WebAssembly.
 
-Usage
+Example
 ----
 
-TODO
+```rust
+#![no_main]
+
+use wasm4 as w4;
+
+struct MyRuntime {
+    count: i32,
+}
+
+// prints "tick" every second
+impl w4::rt::Runtime for MyRuntime {
+    fn start(_: w4::rt::Resources) -> Self {
+        MyRuntime { count: 0 }
+    }
+
+    fn update(&mut self) {
+        if self.count % 60 == 0 {
+            w4::trace("tick");
+            self.count = 0;
+        }
+        self.count += 1;
+    }
+}
+
+w4::main! { MyRuntime }
+```