add usage example

README.md

@@ -18,6 +18,45 @@ cargo add ggemini
 
 * [Documentation](https://docs.rs/ggemini/latest/ggemini/)
 
+### Example
+
+``` rust
+use gtk::gio::*;
+use gtk::glib::*;
+
+use ggemini::client::{
+    connection::{
+        response::meta::{Mime, Status},
+        Response,
+    },
+    Client, Error,
+};
+
+fn main() -> ExitCode {
+    Client::new().request_async(
+        Uri::parse("gemini://geminiprotocol.net/", UriFlags::NONE).unwrap(),
+        Priority::DEFAULT,
+        Cancellable::new(),
+        None, // optional `GTlsCertificate`
+        |result: Result<Response, Error>| match result {
+            Ok(response) => {
+                match response.meta.status {
+                    // route by status code
+                    Status::Success => match response.meta.mime {
+                        // handle `GIOStream` by content type
+                        Some(Mime::TextGemini) => todo!(),
+                        _ => todo!(),
+                    },
+                    _ => todo!(),
+                }
+            }
+            Err(e) => todo!("{e}"),
+        },
+    );
+    ExitCode::SUCCESS
+}
+```
+
 ## See also
 
 * [ggemtext](https://github.com/YGGverse/ggemtext) - Glib-oriented Gemtext API