Added supported versions to the README

Author: thinkingserious

CONTRIBUTING.md

@@ -64,7 +64,7 @@ We welcome direct contributions to the php-http-client code base. Thank you!
 
 ##### Prerequisites #####
 
-- PHP 5.6 or greater
+- PHP 5.6 or 7.0
 - [Composer](https://getcomposer.org/)
 
 ##### Initial setup: #####

README.md

@@ -10,6 +10,12 @@ All updates to this library is documented in our [CHANGELOG](https://github.com/
 
 # Installation
 
+## Prerequisites
+
+- PHP version 5.6 or 7.0
+
+## Install with Composer
+
 Add php-http-client to your `composer.json` file. If you are not using [Composer](http://getcomposer.org), you should be. It's an excellent way to manage dependencies in your PHP application.
 
 ```json