(DOC): clearified installation via composer

- added bash command to allow for copy pasting the command
- added link to getcomposer.org for the case somebody doesn't know what composer is

Author: k0pernikus

README.md

@@ -18,7 +18,11 @@ Example of zero-config use with Composer:
 
 If you don't need any custom completion behaviour, you can simply add the completion command to your application:
 
-1. Install `stecman/symfony-console-completion` using composer
+1. Install `stecman/symfony-console-completion` using [composer](https://getcomposer.org/) by running:
+   ```
+   $ composer require stecman/symfony-console-completion
+   ```
+
 2. Add an instance of `CompletionCommand` to your application's `Application::getDefaultCommands()` method:
 
   ```php