doc: Add hello world example to README. (#1369)

README.md

@@ -35,6 +35,52 @@ gpui = "0.2.0"
 gpui-component = "0.2.0"
 ```
 
+### Basic Example
+
+```rs
+use gpui::*;
+use gpui_component::{button::*, *};
+
+pub struct HelloWorld;
+impl Render for HelloWorld {
+    fn render(&mut self, _: &mut Window, _: &mut Context<Self>) -> impl IntoElement {
+        div()
+            .v_flex()
+            .gap_2()
+            .size_full()
+            .items_center()
+            .justify_center()
+            .child("Hello, World!")
+            .child(
+                Button::new("ok")
+                    .primary()
+                    .label("Let's Go!")
+                    .on_click(|_, _, _| println!("Clicked!")),
+            )
+    }
+}
+
+fn main() {
+    let app = Application::new();
+
+    app.run(move |cx| {
+        // This must be called before using any GPUI Component features.
+        gpui_component::init(cx);
+
+        cx.spawn(async move |cx| {
+            cx.open_window(WindowOptions::default(), |window, cx| {
+                let view = cx.new(|_| HelloWorld);
+                // This first level on the window, should be a Root.
+                cx.new(|cx| Root::new(view.into(), window, cx))
+            })?;
+
+            Ok::<_, anyhow::Error>(())
+        })
+        .detach();
+    });
+}
+```
+
 ### WebView
 
 > Still early and experimental; there are a lot of limitations.