Add requirements to readme

Author: Adam Kalsey

readme.markdown

@@ -3,6 +3,12 @@ Overview
 
 TropoPHP is a set of PHP classes for working with [Tropo's cloud communication service](http://tropo.com/). Tropo allows a developer to create applications that run over the phone, IM, SMS, and Twitter using web technologies. This library communicates with Tropo over JSON.
 
+Requirements
+============
+
+ * PHP 5.3.0 or greater
+ * PHP Notices disabled (All error reporting disabled is recommended for production use)
+
 Usage
 =====