Update README.md

README.md

@@ -6,6 +6,8 @@ The class also allows you to query servers using RCON although this only works f
 
 [Minecraft](http://www.minecraft.net) also uses Source RCON protocol, and this means you can use this class to send commands to your minecraft server while having engine set to Source engine.
 
+**This library requires a [modern version](https://php.net/supported-versions.php) of PHP, so make sure you are using at least PHP 5.4 or newer.**
+
 ### Protocol specifications can be found over at VDC
 * https://developer.valvesoftware.com/wiki/Server_queries
 * https://developer.valvesoftware.com/wiki/Source_RCON_Protocol