Improve installation instructions

Author: ddeboer

README.md

@@ -16,12 +16,17 @@ This library adds:
 Installation
 ------------
 
-Install the library via [Composer](https://getcomposer.org):
+The recommended way to install the Transcoder library is through 
+[Composer](http://getcomposer.org):
 
 ```bash
 $ composer require ddeboer/transcoder
 ```
 
+This command requires you to have Composer installed globally, as explained
+in the [installation chapter](https://getcomposer.org/doc/00-intro.md)
+of the Composer documentation.
+
 Usage
 -----