docs: Added using by slim doc

docs/using-by-slim.md

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+# Using by Slim framework
+
+`medz/cors` package supported [Slim framework 3](https://github.com/slimphp/Slim).
+
+## Configure
+
+See 👉[Configure example](https://github.com/medz/cors#configure).
+
+## Using
+
+Slim frameowk supported class name is `Medz\Cors\Slim\Cors`, Simple use:
+
+```php
+$app = new Slim\App();
+$app->add(new Medz\Cors\Slim\Cors);
+```
+
+> 💡Slim framework middleware using see https://www.slimframework.com/docs/v3/concepts/middleware.html
+
+### Using Settings
+
+```php
+$settings = [
+ // The configure see https://github.com/medz/cors#configure
+];
+$app = new Slim\App();
+$app->add(new Medz\Cors\Slim\Cors($settings));
+```
+
+### Using custom CORS instance
+
+```php
+$cors = // Todo, The instance is \Medz\Cors\CorsInterface::class
+$app = new Slim\App();
+$app->add(new Medz\Cors\Slim\Cors($cors));
+```
+
+## CORS middleware `__construct`
+
+- payload: This is Medz\Cors\CorsInterface or array or null
+- append: The type is boolean, If using `true`, All request set CORS to response hraders.