Added the requirements in README

Author: ezimuel

README.md

@@ -11,6 +11,11 @@ This module provides data structures and rendering for the API-Problem format.
 - [Problem Details for HTTP APIs](http://tools.ietf.org/html/draft-nottingham-http-problem-06),
   used for reporting API problems.
 
+Requirements
+------------
+  
+Please see the [composer.json](composer.json) file.
+
 Installation
 ------------