add documentation

README.md

@@ -3,7 +3,7 @@
 [![Build Status](https://github.com/innmind/async-http-server/workflows/CI/badge.svg?branch=main)](https://github.com/innmind/async-http-server/actions?query=workflow%3ACI)
 [![Type Coverage](https://shepherd.dev/github/innmind/async-http-server/coverage.svg)](https://shepherd.dev/github/innmind/async-http-server)
 
-Description
+Experimental async HTTP server built on top of `Fiber`s.
 
 ## Installation
 
@@ -13,4 +13,48 @@ composer require innmind/async-http-server
 
 ## Usage
 
-Todo
+```php
+# server.php
+<?php
+declare(strict_types=1);
+
+require 'path/to/vendor/autoload.php';
+
+use Innmind\Async\HttpServer\Main;
+use Innmind\OperatingSystem\OperatingSystem;
+use Innmind\Http\Message\{
+    ServerRequest,
+    Response,
+    StatusCode,
+};
+use Innmind\Filesystem\Name;
+use Innmind\Url\Path;
+
+new class extends Main {
+    protected static function handle(ServerRequest $request, OperatingSystem $os): Response
+    {
+        return $os
+            ->filesystem()
+            ->mount(Path::of('somewhere/'))
+            ->get(Name::of('some-file'))
+            ->match(
+                static fn($file) => new Response\Response(
+                    StatusCode::ok,
+                    $request->protocolVersion(),
+                    null,
+                    $file->content(),
+                ),
+                static fn() => new Response\Response(
+                    StatusCode::notFound,
+                    $request->protocolVersion(),
+                ),
+            );
+    }
+};
+```
+
+You can run this server via the command `php server.php`. By default the server is exposed on the port `8080`.
+
+This example will return the content of the file `somewhere/some-file` if it exists on the filesystem otherwise it will respond with a `404 not found`.
+
+The asynchronicity of this program is handled by the `OperatingSystem` abstraction meaning you can write code _as if_ it was synchronous.