Added README for ReactPHP (#18)

Author: Nyholm

README.md

@@ -0,0 +1,42 @@
+# ReactPHP Runtime
+
+A runtime for [ReactPHP](https://reactphp.org/)
+
+## Installation
+
+```
+composer require runtime/react
+```
+
+## Usage
+
+Define the environment variable `APP_RUNTIME` for your application.
+
+```
+APP_RUNTIME=Runtime\React\Runtime
+```
+
+### PSR-15
+
+```php
+// public/index.php
+
+use Psr\Http\Server\RequestHandlerInterface;
+use Psr\Http\Message\ServerRequestInterface;
+use Psr\Http\Message\ResponseInterface;
+use Nyholm\Psr7;
+
+require_once dirname(__DIR__).'/vendor/autoload_runtime.php';
+
+class Application implements RequestHandlerInterface {
+    // ...
+    public function handle(ServerRequestInterface $request): ResponseInterface
+    {
+        return new Psr7\Response(200, [], 'PSR-15');
+    }
+}
+
+return function (array $context) {
+    return new Application($context['APP_ENV'] ?? 'dev');
+};
+```