add info on PHP version, cf. #23

Minishlink · Minishlink · commit 26490e9e883d · 2016-07-29T00:46:00.000+02:00
diff --git a/README.md b/README.md
@@ -14,6 +14,8 @@ As it is standardized, you don't have to worry about what server type it relies
 
 Notifications with payloads are supported with this library on Firefox 46+ and Chrome 50+.
 
+You'll need at least PHP 5.6, and PHP 7.1 is recommended for much better performance.
+
 ```php
 <?php
 
