add requirements

Author: chobie

README.md

@@ -5,6 +5,10 @@ php-git2 is a PHP bindings to the libgit2 linkable C Git library.
 * API Documentation: http://libgit2.github.com/libgit2/#v0.20.0 (also see Signature conversions section)
 * IRC: #php-git on irc.freenode.net.
 
+## Requirements
+
+PHP 5.3 above
+
 ## Status
 
 0.3.0 Alpha (switching to functions)