update requirements in readme

Author: dbu

README.md

@@ -24,7 +24,8 @@ and licensed under the [MIT License](LICENSE).
 
 ## Requirements
 
-* The Symfony Routing component (>= 2.2.0)
+* PHP 5.6 / 7
+* The Symfony Routing component (2.2.0 - 3.\*)
 * See also the `require` section of [composer.json](composer.json)